Mastering Freshwater Aquarium Ecosystems
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Adjusting Water Chemistry

In some cases you will not need to adjust the water chemistry in a freshwater aquarium. Often the water that comes from the tap is adequate for the fish and plants you may desire to keep. If the fish and plants you want to keep require different water chemistry than what you have coming out of your faucet, you will need to add or remove major trace elements.

Whenever you add chemicals to the to adjust the chemistry, first dilute it in a cup of water and then slowly pour into the aquarium in a high flow area to help disperse it quickly. Dumping raw chemicals in the tank can damage live plants and kill beneficial bacteria. Water chemistry should always be adjusted slowly over the course of several days or weeks if a major change is required. Keep a log of what you add to the water to adjust the chemistry, so you will know how much to add to new water when making water changes.

Large quick adjustments of the water chemistry can cause a condition in fish known as osmotic shock and/or pH shock. Osmotic and pH shock can kill fish in three days or less. The symptoms include; resting on the bottom of the tank, and abnormal heavy respiration. Fish that display these symptoms can be saved by returning them to aquarium conditions they were acclimated to before making the adjustment. It can take several days for a fish to recover from osmotic shock, so it may be necessary to use an isolation container to protect the fish from other inhabitants.


pH/alkalinity can be adjusted up by adding sodium bicarbonate (baking soda), sodium carbonate (soda ash, washing soda, pool pH increaser), or a pH increaser from many of the aquarium product manufacturers. For most freshwater aquarium sodium bicarbonate is the primary chemical used to increase the pH. As long as the target pH is not greater than 7.8, sodium bicarbonate would be the chemical of choice. If you are going to keep Lake Malawi, or Lake Tanganyika fish which require very high pH range, sodium carbonate would be the primary chemical of choice. When using sodium carbonate, use only a quarter of the amount you would use with sodium bicarbonate as it has a greater effect on pH, and will help maintain a higher than 7.8 pH.

Adding 1 teaspoon of sodium bicarbonate per 10 gallons (40 liters) will raise the TDS approximately 90 ppm. Since nitrate is an acid, the pH will vary depending on how much nitrate is in the aquarium.

pH Increasers

Sodium carbonate (pH UP) and sodium bicarbonate (Baking Soda) are the two most common chemicals used to increase pH in aquariums.

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