Water Chemistry Testing

Establishing a baseline pH, KH, and GH is essential before adjusting water chemistry. Testing tap pH requires allowing it to gas off any excess CO₂, O₂, or other gasses that may temporarily affect the pH. Filling a glass with water and aerating for 10 minutes will allow for gas equilibrium before the water is tested. Allowing tap water to gas off can significantly affect the pH test results (-0.9 pH difference [8.7 to 7.8] with the author's tap water).

Adding minerals to the water is an easy process, and it does not take a lot of dry chemicals to raise the levels. Typically, most hobbyists will use 1 level teaspoon or less of a compound per 10 gallons (4 L) to raise the concentration. The initial purchase of the dry chemicals will last most hobbyists for years. Typically, general hardness and carbonate hardness are adjusted when completing a water change. Carbonate hardness may need to be adjusted between water changes depending on biomass and how frequently water changes are done.

Spoons used to measure can vary in the amount of compound they hold. For this ebook, all measurements are done with KitchenAid Classic Measuring Spoons, available in many stores and websites. All spoons are level and not compacted.

The grain size can vary with dry compounds depending on the source. The source is stated for all compounds used for providing amount recommendation in this ebook.

Chemicals used to adjust water chemistry can hydrate from humidity in the air. When the chemicals hydrate, they can add weight. Therefore any gram/milligram measurements provided per one teaspoon are not precise measurements. The same amount by volume of the compound could weigh less or more for other hobbyists. Keep all chemicals in a sealed container when not in use to prevent excessive hydration.

The volume of a system must be known before adjusting the water chemistry. Typically when adding minerals to the system for the first time, the hobbyist must calculate the volume. The volume includes the inside dimensions of the aquarium plus the amount of water in external filters and tubing minus substrate and decor. This will provide a good enough estimate to make adjustments.

The volume of the aquarium in gallons is calculated on the length times width times height in inches, divided by 231 will equal gallons.

Volume = Length x Width x Height / 231

For liters, measure in centimeters instead of inches, and divide by 1000 will provide liters.

Volume = Length x Width x Height / 1000

When doing water changes, compounds are added based on the volume of the new water. GH typically remains relatively stable and only gets used very slowly. KH may require an additional amount of a compound because plants and bacteria use it at much higher rates than GH.

GH and KH measurements in this ebook are done with API liquid test kits. TDS is measured with Hanna Instruments TDS/Temp handheld meter.

For high pH systems (African rift lakes) or low pH (below 6.0), it is recommended that hobbyists invest in a pH monitor to determine the value accurately. For observing pH changes in the system from added compounds, a Milwaukee pH digital monitor was used.

pH values for Lake Tanganyika fish can exceed the top range of API's liquid High Range pH test kit (8.8), and low KH systems (below 1 dKH) will often measure below 6.0.

All compound quantity recommendations to raise a value were developed using 5-gallon bucket tests and repeated tests to validate results.

A milligram scale (Fuzion Digital Milligram Scale 50/0.001 g) was used to observe the variance in teaspoons and to keep actual weight consistent. All water chemistry results were logged.

Any adjustments in chemicals added to the system should be logged. Logging helps determine trends and if the change to the system is having a positive or negative effect.

General hardness (or total hardness) is the amount of dissolved calcium and magnesium in the water. Tap water using groundwater as a source should be concerned with water hardness. As rainwater moves through soil and rock, it dissolves small amounts of naturally-occurring minerals and carries them into the groundwater. Rainwater is an excellent solvent for calcium and magnesium, so hard water may be delivered to your home if the minerals are present in the soil around a well water supply. Surface water (rivers, lakes, streams, creeks) will have lower GH in the local area than groundwater.

Calcium and magnesium are essential nutrients for aquatic plants and animals. Aquatic animals can get calcium and magnesium from water and food. Most natural freshwater systems have a general hardness and alkalinity of similar concentration and typically range from less than 5 to over 150 ppm. Many aquatic plant species can do well in a relatively low concentration (< 10 ppm) of calcium and magnesium. Still, hobbyists must consider the animal's needs as their long-term health is often related to GH. Many species of fish and plants kept in aquariums will benefit from very hard water.

Hobbyists can add calcium and magnesium to the water with calcium chloride, magnesium chloride, and magnesium sulfate. The chemicals are available from aquarium stores, retail websites, and bulk chemical websites. These chemicals have other uses, so you can find them available from non-aquarium-related businesses. These chemicals are commonly used in reef aquariums.

A system's calcium/magnesium ratio (Ca/Mg ratio) will directly affect the amount of bicarbonate/carbonate in the solution. Higher ratios of magnesium will support higher levels of carbonate.

Aquarium product manufacturers often market products that have calcium and magnesium mixed in a single container. The ratios in these products are typically formulated to target specific water chemistry, and the recommended dosage is for distilled or reverse osmosis water. Separately adding calcium and magnesium gives hobbyists greater control over the Ca/Mg ratio. Adding calcium to tap water before allowing the magnesium to increase could result in calcium precipitating out of the solution with carbonate in the water.

The molar mass (molecular weight, g/mol) and the atomic mass unit (AMU) of the calcium and magnesium compounds we use to raise GH must be factored in when targeting a specific ratio. Hobbyists can use the Periodic Table of the Elements to provide the “average atomic mass” of the atoms and calculate the molar mass. Once the molar mass is known, the percentage of calcium or magnesium in a compound can be calculated.

Example, the molar mass of magnesium chloride (MgCl₂) is 95.21:

Atomic mass of Mg = 24.31
Atomic mass of Cl = 35.45

There are 2 atoms of Cl, multiply Cl mass by 2 first, then add the Mg mass.

Mg + Cl(2) = 95.21

24.31 + 35.45(2) = 95.21

Only 25.5% of the molar mass of MgCl₂ is magnesium:

24.31/95.21 = .2553

If you add a teaspoon of magnesium chloride to the system, only 25% is magnesium (which adds to GH), and the rest is chloride.

Hobbyists should dissolve all chemicals in a container of water before adding them to the system in a high-flow area. Dumping undissolved chemicals into the system will kill microbes in the substrate and can damage plants.

The order in which minerals are added to the system is important to prevent the precipitation of calcium carbonate:

1. Magnesium
2. Calcium
3. Bicarbonate
4. Carbonate (high pH systems only)

Magnesium

Magnesium (Mg) is essential to aquatic life. Magnesium can be toxic in the water when calcium levels are too low (1 part calcium to 9 parts magnesium)^[20].

Life in a closed system slowly uses the magnesium in the water. The magnesium level in most tap water is sufficient for aquatic plants. Some have suggested that 2 ppm of magnesium is sufficient for plants. Typically, additional magnesium is added to a system for the animals and is at least 20 ppm.

The amount of magnesium in marine systems (about 1300 ppm) plays a crucial role in maintaining alkalinity and preventing carbonate from combining with calcium (seawater concentration is around 400 ppm). When the precipitation of calcium carbonate happens, it lowers the alkalinity (dropping pH) and the amount of calcium available for coral. Reef hobbyists often adjust the magnesium to around 1350 ppm to prevent calcium carbonate precipitation. The ratio of calcium to magnesium in freshwater systems is more complicated since the water chemistry is highly variable.

Acids in freshwater dissolve calcium carbonate and can prevent precipitation from happening in most systems, but not all. It is still possible to have calcium carbonate precipitation in freshwater systems, and when it happens, it can coat surfaces in the system with deposits. Some observations in aquaculture systems suggest that calcium carbonate precipitation could be harmful to invertebrates^[21]. Hobbyists can prevent calcium carbonate precipitation by increasing the amount of magnesium in the water.

Scientific research on the ratios of calcium to magnesium in freshwater to avoid the precipitation of calcium carbonate has not been done or published at the time of this writing. The author's general guidance for freshwater systems based on experience is to maintain a one-to-one ratio of calcium to magnesium for systems at or below a pH of 7.8.

For systems with a pH above 7.8 maintained with sodium carbonate, magnesium must be at a much higher Ca/Mg ratio. Lake Tanganyika has about 4 parts magnesium to 1 part calcium that helps maintain a pH between 8.46 to 9.08.

Hobbyists can easily observe precipitation when pre-dissolved sodium carbonate is added to the system, and it immediately turns white. Precipitation indicates that the ratio of magnesium to calcium should be increased.

The hobbyists can use magnesium chloride (preferred compound) or magnesium sulfate to increase the magnesium.

Magnesium Chloride (MgCl₂)

Magnesium chloride is the preferred chemical to raise magnesium levels in aquatic systems due to the beneficial effects the chloride ions have on preventing the intake of nitrite through the fish's gills. Research has pointed out that MgCl₂ was more effective and less toxic than magnesium sulfate (MgSO₄) in maintaining optimal aquaculture of scallops^[22]. Anecdotal evidence and additional research that compare magnesium chloride and magnesium sulfate are showing magnesium chloride is more beneficial and eliminates some of the side effects of magnesium sulfate.

Magnesium chloride is used in the commercial production of textiles, snow and ice melt, paper manufacturing, medicine, and cement, to name a few. Any aquarium reef retailer will often have multiple bands of magnesium chloride. Many forms of magnesium chloride can be bought already diluted in water.

Adjusting the magnesium level is the first step to increasing the mineral content of the system water. One level teaspoon of magnesium chloride, about 4 grams (source: Bulk Reef Supply) per 10 gallons (4 L), will raise the GH by about 3.5 dGH and the TDS by 70 ppm.

25.5% of the molar mass of MgCl₂ is magnesium.

The molar mass of magnesium chloride (MgCl₂) is 95.21:

Atomic mass of Mg = 24.31
Atomic mass of Cl = 35.45

There are 2 atoms of Cl. First, multiply Cl atomic mass by 2, then add the Mg atomic mass.

Mg + Cl(2) = 95.21

24.31 + 35.45(2) = 95.21

24.31/95.21 = .2553

If you add a teaspoon of magnesium chloride to the system, only 25.5% is magnesium which adds to GH, and the rest is chloride.

Magnesium Sulfate (MgSO₄)

Magnesium sulfate, sometimes spelled "sulphate" (Epsom salt), is often used to raise the magnesium level in freshwater and marine systems. Research shows sulfate has a laxative effect on animals (livestock and humans). Unfortunately, little (if any) research has been done on the impact on fish. Research has shown a relationship between hardness and chloride concentrations and sulfate toxicity. Some studies suggest that elevated levels of sulfate could affect reproduction. More research needs to be done on how sulfate may affect reproduction in aquatic animals kept in aquariums or aquaculture^[23].

Some hobbyists recommend adding magnesium sulfate to treat swim bladder issues and dropsy, but no scientific studies support its uses as an effective treatment. Magnesium sulfate taken with food may help treat Spironucleus (often incorrectly called Hexamita). A 3% Epsom salt feed showed significant improvement in rainbow trout (Oncorhynchus mykiss) diagnosed with Spironucleus^[24].

Magnesium sulfate can be used to raise the magnesium level in the system. Still, because chloride ions have shown to have beneficial effects on fish, magnesium chloride would be the better choice.

Magnesium sulfate is used in industrial and medical treatments. Epsom salt is commonly available in grocery stores and pharmacies. Any aquarium reef retailer will often have multiple bands of magnesium sulfate. Many forms of magnesium sulfate can be bought already diluted in water.

One level teaspoon of magnesium sulfate (source: Bulk Reef Supply), about 4 grams per 10 gallons (4 L), will raise the GH by about 4.5 dGH and the TDS by 70 ppm.

20.2% of the molar mass of MgSO₄ is magnesium.

The molar mass of magnesium sulfate (MgSO₄) is 120.38:

Atomic mass of Mg = 24.31
Atomic mass of S = 32.07
Atomic mass of O = 16.00

There are 4 atoms of O. First, Multiply O atomic mass by 4, then add the Mg and S atomic mass.

Mg + S + O(4) = 120.38

24.31+32.07+16(4) = 120.38

24.31/120.38 = .2019

If you add a teaspoon of magnesium sulfate to the system, only 20.2% is magnesium, which adds to GH, and the rest is sulfur and oxygen.

Calcium

Calcium Chloride (CaCl₂)

Calcium (Ca) restricts the rise in pH when photosynthesis rates are high. When plants deplete the water of free carbon dioxide, they can use bicarbonate as a carbon source. But when using bicarbonate (HCO₃), plants release carbonate (CO₃), which causes pH to increase. Calcium ions react at elevated pH to precipitate carbonate ions as calcium carbonate, and this reaction reduces the amount of carbonate in the water to maintain KH and pH.

Calcium affects the toxicity of trace metals to fish and other aquatic animal species. The presence of calcium blocks the uptake of metal ions across the gills, thereby increasing the dissolved concentration of metals required to cause a toxic effect^[25]. Studies of striped bass, channel catfish, and white perch have shown calcium increased the hatch rate of eggs^[26]. Studies have concluded that up to 50% of salmonids calcium need comes through the gills from dissolved calcium in the water^[27]. Calcium deficiency can result in skeletal deformities.

Aquatic plants should have a minimum of 2 ppm calcium in the water. In most systems, calcium levels maintained for fish are much higher than the minimum required for plants. Many aquatic plant species will only thrive in moderately hard to very hard water.

Calcium chloride (CaCl₂) is the preferred chemical to increase calcium in marine and freshwater systems. Calcium and chloride have beneficial effects on fish health.

Calcium chloride is used for many commercial applications, including pool hardness increaser, Snow Joe Melt Calcium Chloride Pellet, Dow Flake, and aquariums. Most aquarium reef retailers will have multiple bands of calcium chloride. Many forms of calcium chloride can be bought already diluted in water.

Adjusting the calcium level is the second step to increasing the mineral content of the system water. Calcium chloride will heat up when added to water. One level teaspoon, about 4.5 grams of calcium chloride (source: Bulk Reef Supply) per 10 gallons (4 L) will raise the GH by about 5.5 dGH, and the TDS by 105 ppm.

36.1% of the molar mass of CaCl₂ is calcium.

The molar mass of calcium chloride (CaCl₂) is 110.98:

Atomic mass of Ca = 40.08
Atomic mass of Cl = 35.45

There are 2 atoms of Cl, multiply Cl atomic mass by 2 first, then add the Ca atomic mass.

Ca+ Cl(2) = 110.98

40.08+35.45(2) = 110.98

40.08/110.98 = .3611

If you add a teaspoon of calcium chloride to the system, only 36% is calcium (which adds to GH), and the rest is chloride.

Carbonate Hardness (KH)

Carbonate hardness (KH) measures the carbonate and bicarbonate anions. The reason for the “K” in KH is that the original measure comes from the German word for carbonate, which starts with a “K.” In the aquarium hobby, the carbonate hardness is commonly measured with degrees of hardness and expressed with the acronym “dKH.” One degree of carbonate hardness equals 17.86 ppm.

The stability of KH is directly related to the Ca/Mg ratio. Higher ratios of magnesium can support higher levels of KH and pH. Low magnesium levels can cause the system's KH and GH to drop.

In most natural bodies of water, KH is often lower than GH, but many exceptions exist (Lake Tanganyika). Lake Tanganyika has a GH of around 11 dGH and 18 to 20 dKH.

The KH will directly affect the stability and range (day/night fluctuation) of the pH. A general guideline for KH is to keep it at or above 4 dKH for stability. The 4 dKH guideline is fine for helping maintain a healthy nitrogen cycle, but it is not the ideal level for all systems.

Adjusting the KH with sodium bicarbonate and sodium carbonate does not add to GH. KH will need to be tested after adding a KH buffer. It can take 24 to 48 hours for KH levels to stabilize after adding a buffer. Another minor adjustment may be required to meet the desired KH range. Bicarbonate and carbonate have very low electrical conductivity (EC) and will not show significant ppm on the TDS meter. The majority of the increase in TDS is caused by sodium (high EC). The TDS ppm does not increase as much as the KH ppm.

For systems where the pH needs to be maintained at very low levels (6 or below), below 1 dKH is recommended. For low KH systems, it is recommended that the hobbyist invest in a Hanna Instruments Freshwater Alkalinity Colorimeter, which measures in ppm and gives a more precise result.

For Lake Tanganyika fish, an 18 dKH or greater is recommended. The relative concentrations of carbon dioxide, oxygen, bicarbonate, and carbonate influence pH. Carbonate is the dominant species in Lake Tanganyika's water which helps support the high pH in the 8.46 to 9.08 range^[28]. For Lake Malawi, a 10 dKH or greater is recommended^[29].

Bicarbonate

Sodium Bicarbonate (NaHCO₃)

Baking Soda (sodium bicarbonate) NaHCO₃ is the primary ingredient in most pH increasers targeted to aquarium hobbyists. Bicarbonate is a major molecule that provides most of the KH in freshwater systems where the target pH is 7.8 or below.

When aquatic plants use bicarbonate, they release carbonate, raising the pH. If the magnesium level in the system is low, carbonate can combine with calcium and precipitate out of the solution, lowering KH and GH. Acids in a freshwater system can dissolve the calcium carbonate precipitate, raising the KH and GH.

When the KH is very low (< 1 dKH), the pH can be at or below 6. When the pH is at 6 or below, it is common to see the ammonium concentration creep up. The lack of KH in a system is the cause of "Old Tank Syndrome." While low KH does affect the nitrogen cycle, it is possible to maintain a low KH and pH system, but it requires a shift in the microbe species to maintain a nitrogen cycle. Low KH and pH systems are covered later in this ebook.

For a system maintained at a pH above 6.0, KH is like a gas tank for the nitrogen cycle, and bicarbonate is the fuel. Bacteria and some aquatic plants use bicarbonate over time, and the heavier a system is stocked with animals and plants, the faster the KH will be drawn down. Frequent water changes can negate the need to add more KH to the system. Hobbyists who do not like frequent water changes may need to add sodium bicarbonate on a regular schedule to maintain a target KH range.

Adjusting the bicarbonate/KH level is the third step to increasing the mineral content of the system water. One level teaspoon of about 5.8 grams of NaHCO₃ (source: Arm & Hammer) per 10 gallons (4 L) will raise the KH by about 5 dKH and TDS by 80 ppm.

72.6% of the molar mass of NaHCO₃ is bicarbonate (HCO₃).

The molar mass of sodium bicarbonate (NaHCO₃) is 84.01:

Atomic mass of Na = 22.99
Atomic mass of H = 1.01
Atomic mass of C = 12.01
Atomic mass of O = 16.00

There are 3 atoms of O, multiply the O atomic mass by 3 first, then add the C atomic mass, H atomic mass, and Na atomic mass.

Na+ H + C + O(3) = 84.01

22.99+1.01+12.01+16(3) = 84.01

22.99/84.01 = .2736

If you add a teaspoon of sodium bicarbonate to the system, 72.6% is HCO₃ (which adds to KH [low electrical conductance]), and 27.4% is sodium which adds to TDS because of its electrical conductance.

Carbonate

Sodium carbonate (Na₂CO₃)

Soda Ash (sodium carbonate) Na₂CO₃ is often used to raise the pH in high pH (over 7.8) freshwater and marine systems. Sodium carbonate is used to help maintain a pH above 7.8. Carbonate is also included in the KH test result. Sodium carbonate has a much greater effect on raising the pH than sodium bicarbonate.

In most freshwater systems, Na₂CO₃ is not needed. It is best to add very small quantities of 500 mg (1/10 of a level teaspoon) per 10 gallons (4 L) per day. Carbonate takes time to react with acids (i.e., CO₂). Test the pH 24 hours after adding to the system to see if your target range has been achieved.

Carbonate can naturally occur; when plants use bicarbonate, they produce carbonate. Carbonate raises the pH many times more than bicarbonate. The ratio of carbonate to bicarbonate will affect pH. In high pH systems (above 7.8), the ratio of carbonate to bicarbonate will go up as the system's pH increases.

Keep in mind the stability of carbonate in solution is directly related to the Ca/Mg ratio. A white precipitate will occur if the system's magnesium level is too low. In the case of extreme ionic imbalance, the water will turn cloudy white for more than 15 minutes. The calcium carbonate precipitate will lower GH and KH.

The residue of the calcium carbonate precipitate has the consistency of chalk powder. Some calcium carbonate precipitate is common even when the system's Ca/Mg ratio is correct. When Na₂CO₃ is added to the system, it can quickly precipitate some calcium carbonate at the location where it is added.

Sodium carbonate is used in making glass, pool pH increasers, detergents, and cleansers.

Adjusting the carbonate/KH level is the fourth step (high pH systems only) to increase the mineral content of the system water. One level teaspoon or about 5.5 grams of Na₂CO₃ (source: Pool Time) per 10 gallons (4 L) will raise the KH by about 8 dKH, and TDS by 110 ppm. One teaspoon of Na₂CO₃ will raise the KH 3 dKH more than the same amount of NaHCO₃.

56.6% of the molar mass of Na₂CO₃ is carbonate (CO₃).

The molar mass of sodium carbonate (Na₂CO₃) is 105.99:

Atomic mass of Na = 22.99
Atomic mass of C = 12.01
Atomic mass of O = 16.00

There are 2 atoms of Na, multiply the Na atomic mass by 2, 3 atoms of O, multiply the O atomic mass by 3, then add the C atomic mass, total Na atomic mass, and total O atomic mass.

Na(2) + C + O(3) = 105.99

22.99(2)+12.01+16(3) = 105.99

22.99(2)/105.99 = .4338

If you add a teaspoon of sodium carbonate to the system, 56.6% is CO₃ (which adds to KH [low electrical conductance]), and 43.4% is sodium which adds to TDS because of its electrical conductance.

Carbon Dioxide

Carbonic acid (H₂CO₃) is formed in the system when carbon dioxide (CO₂) combines with carbonate. This interaction causes the pH to drop. At night plants stop using CO₂ and start using oxygen, and produce CO₂. This process causes CO₂ to rise naturally and the pH to drop. When a planted aquarium lights are turned on, the reverse happens, CO₂ is used by plants, and they produce oxygen, which causes the pH to rise.

Testing the pH in the morning before the lights turn on will show the pH is much lower than in the late evening before the lights go off. It is common for systems to fluctuate as much as one full pH point (8.0 to 9.0) or more. This natural pH fluctuation can be minimized by strong surface agitation.

In systems that do not have strong surface agitation, the day/night cycle can be problematic for fish that need a high oxygen level in the water or if treating the system with medication. Medications can lower the oxygen-carrying capacity of water. Elevating the water temperature for disease treatment will also lower the oxygen-carrying capacity of water. While the oxygen level during the day cycle is adequate, at night, the plants can use so much oxygen they can suffocate fish. Ensuring the system has strong surface agitation at night will neutralize the low oxygen risk.

Calcium Carbonate

Hobbyists often use calcium carbonate (CaCO₃) to help maintain KH. Typically aragonite (coral) sand or gravel is added to the substrate or filter. Adding limestone, calcite, and dolomite are other methods to maintain KH. Acids in freshwater systems can slowly dissolve CaCO₃, and they will add calcium and carbonate to the water. While this will help prevent a system from crashing the nitrogen cycle, it also adds GH and KH slowly.

Relying on his method makes it impossible to maintain consistent water chemistry. Water changes in systems with calcium carbonate material will lower the GH and KH. Depending on the source water chemistry, this can drastically lower the GH and KH values of the aquarium. Quick changes in osmotic pressure can stress and even kill the animals in the aquarium. Systems with calcium carbonate should always have the water chemistry adjusted after a water change to prevent osmotic shock.

For hobbyists that maintain systems with Gulf Coast USA, Mexico, Central America, Northern South America, Caribbean Islands, Lake Malawi, and Lake Tanganyika fish, adding substrate that can buffer the pH and add to KH is acceptable but not required.

Aquatic plants grow in calcium carbonate sand in areas of the world where limestone (fossil coral) is a major part of the geology.

Sodium Chloride

Salt (sodium chloride) NaCl can be beneficial at low levels. When the KH is increased in aquariums, sodium is added with bicarbonate and carbonate. Chloride also has beneficial effects. Chloride is added to the system with magnesium chloride and calcium chloride to increase GH. The chemicals used to raise KH and GH add enough sodium and chloride for healthy fish and plants.

Sodium (Na), while beneficial at low levels, high concentrations disturb and inhibits various physiological processes and plant growth. The chemical similarities between potassium (K) and Na allow some of the functions of potassium to be undertaken by sodium. High concentrations of cytosolic sodium (fluid in cells) are toxic to plants. The major role of Na is considered to be the regulation of turgor pressure (hydrostatic pressure) and cell expansion^[30].

Salt stress significantly influences various ion homeostasis, such as calcium, phosphate, nitrate, and results in the reallocation of these ions to create adjusted homeostasis. Due to the chemical similarity between Na and K, K homeostasis is severely influenced during salt stress. High sodium concentrations in plants often cause phosphate deficiency symptoms. Sodium can interrupt many physiological processes mediated by phosphate, such as protein synthesis and enzymatic reaction.

Freshwater aquatic plants (macrophytes) that inhabit freshwaters display efficient tolerance to salinity levels between 5,000 and 10,000 ppm, and only a few species display tolerance higher than 10,000 ppm. Widespread freshwater aquatic plants demonstrated a salinity tolerance of approximately 5,000 ppm. Salinities above 1,000 ppm can negatively affect the physiology and diversity of freshwater aquatic plants and cause long-term changes in freshwater ecosystems. Submerged, emergent, and floating aquatic plants showed no significant difference in salinity tolerance^[31].

Salt should be avoided in planted aquariums. NaCl is best used in a quarantine system with fish that are salt tolerant. Salt is an excellent medication that can kill many parasites, bacteria, and fungus species. NaCl does not add to GH or KH, but it substantially affects TDS because of its EC. One level teaspoon of NaCl will raise the TDS by about 170 ppm per 10 gallons (4 L) of water. Sodium chloride has many common names, table salt, solar salt, rock salt, pool salt, pond salt, and aquarium salt. Some table salts contain iodine and anticaking agents.

A common freshwater aquarium myth is that the iodine and anti-caking agents harm fish. The anti-caking agent in Morten table salt is calcium silicate, which is aquarium safe. The tiny trace amount of iodine in table salt is more likely to be beneficial. Many aquarium manufacturers make iodine supplements for aquariums, also known as Lugol's solution. Scientific research has shown that iodine supplementation improves shrimp's growth (Penaeus chinensis) significantly and raises survival rates^[32]. Multiple studies have shown positive growth of plants with iodine supplementation.

Do not confuse salt sodium chloride with marine salt. Marine salt contains magnesium, calcium, bicarbonate, carbonate, and many other trace minerals.

Freshwater nitrifying bacteria can tolerate up to 8 parts per thousand (ppt) (specific gravity 1.006) of sodium chloride.

You can use a marine hydrometer to check the salinity in a quarantine system.

Sodium Thiosulfate

Sodium thiosulfate (Na₂S₂O₃) is the active ingredient in all pond, aquarium, and pool water conditioners that dechlorinate tap water. Sodium thiosulfate neutralizes chlorine and chloramine. Sodium thiosulfate can be bought in bulk and is often called sodium thiosulfate pentahydrate, Na₂S₂O₃·₅H₂O.

Sodium thiosulfate is also used as a photographic fixer, in medicine, paper production, and gold mining.

The tiny amount of sodium thiosulfate used to dechlorinate does not add significant TDS to the system.

Total Dissolved Solids

The total dissolved solids (TDS) meter measures the number of solids in a solution. TDS is often measured with a handheld meter, and the results are displayed in ppm.

The TDS meter is a tool that can provide a base (starting point) ppm and a result ppm after adding a compound. The meter's accuracy is highly variable depending on the different species of atoms in the solution.

KH plus GH does not equal TDS. The TDS meter measures electrical conductivity (EC), which is then converted to ppm based on the EC of salt (sodium chloride). A sodium chloride (NaCl) solution is used to calibrate TDS meters. Sodium chloride and other compounds hobbyists add to their systems have a different EC value than NaCl. Sodium (Na) has an EC value of 0.21 106/cm Ω^[33]. Because of the difference in EC, adding different compounds of the same quantity will significantly affect how much ppm will be displayed.

Electrical Conductivity of Atoms Used in Water Chemistry

TDS meters are an essential tool to check the efficiency of reverse osmosis (RO) filtration. The effluent of the RO is checked for TDS, if they are higher than expected, maintenance is required.

A TDS meter can also provide a general baseline of the mineral contents of tap water. Tap water TDS can vary greatly over a year, and the ppm should be known before making water chemistry adjustments.

Reverse Osmosis (RO) Filter

Some hobbyists will have too much GH and KH in their source water. Polluted source waters often have high nitrate levels, often greater than 40 ppm. These issues can be remedied by installing a reverse osmosis (RO) filter on the tap water line.

A RO filter will remove TDS and nitrate from the tap water. RO water is very soft, with a TDS often below 5 ppm. With RO water, hobbyists need to add minerals back unless the intent is to make the system water more soft and acidic.

A RO filter will have two lines of water coming out of the filter, wastewater and filtered water. Filtered water can be collected in a large plastic trashcan or another type of reservoir. The collection reservoir should have a float valve installed to prevent overflow and cut off water flow to the filter when the reservoir is full. Wastewater can be used in a garden or sent down the drain.

RO filters will have a sediment filter, carbon filter (remove chlorine), and membrane filter. Some hobbyists also use a deionizing (DI) stage with resins to remove the final small amount of TDS that passes through the RO filter. Filters that use both types of filtration are called RO/DI filtration systems.

Remineralizing RO and RO/DI Water

All adjustments are based on 10 gallons (4 L). Teaspoons can vary slightly. These values were developed using KitchenAid Classic Measuring Spoons. All measurements provided are based on level teaspoons.

Magnesium is the key to supporting a stable KH. If magnesium is too low, calcium will combine with carbonate and precipitate out of the solution.

Magnesium can be toxic if you do not have a high enough calcium level. A Ca/Mg ratio of 1 to 9 has proven toxic in scientific research.

The presence of calcium blocks the uptake of metal ions across the gills, thereby increasing the dissolved concentration of metals required to cause a toxic effect.

One teaspoon of calcium chloride adds more to GH than magnesium chloride. You can add more magnesium chloride or reduce the amount of calcium chloride in your remineralizing formula to support a higher KH.

Bulk Reef Supply is a good source of dry bulk magnesium chloride and calcium chloride.

Keep magnesium chloride and calcium chloride sealed when not in use, as they can become easily hydrated.

These chemicals must be added to RO and RO/DI for water changes. RO and RO/DI water need not be remineralized for topping off a system due to evaporation. Adding all chemicals separately allows hobbyists to control exact ratios based on the plants and fish they keep.

Dilute all chemicals in a cup of water separately, then add to a high-flow area of the system.

The order in which minerals are added to the system is important to prevent the precipitation of calcium carbonate:

1. Magnesium
2. Calcium
3. Bicarbonate
4. Carbonate (high pH systems only)