Water changes are primarily performed to lower the nitrate level in the aquarium. Regular scheduled water changes are often essential to maintaining a healthy aquarium ecosystem. How often you need to do a water change and how much will be determined by how fast nitrate accumulates in the aquarium. You should look at nitrate as being smog in the aquarium. The higher the concentration of nitrate, the greater the pollution.
In a freshwater aquarium you should try keep the nitrate level as close to 0 ppm as possible. For most freshwater aquarium systems, nitrate will accumulate over time and should not be allowed to exceed 100 ppm for more than two weeks.
When conducting a water change on an established aquarium, you should try to match the TDS and pH of the water you are replacing. Matching the TDS is more critical than pH, but ignoring pH can also lead to fish mortality.
When changing the water of the aquarium you should be within 100 ppm TDS after completing the water change. TDS can be adjusted after a water change if necessary. If a radical change in TDS is allowed after a water change, osmotic shock can potentially occur in fish, which can lead to fish mortality if not corrected.
In an established aquarium, pH will drop as nitrate increases. Adding sodium bicarbonate (baking soda) over the course of a week to bring the pH up gradually will help fish make the transition to a higher pH that will typically occur after a water change. Conducting small (25%) partial water changes daily over a week can also help fish adjust to the higher pH slowly. Fish mortality is common when aquarist do complete or near complete water change on an aquarium that has not had its water changed in several months, and the pH has been allowed to get very acidic (due to extremely high nitrate level) for an extended period.
The consistency of water temperature is often given over stated importance when doing a water change. In natural waters, 5 to 10°F (2 to 5°C) difference in temperature is common in different areas of rivers, streams, creeks, ponds and lakes that fish traverse daily. Fish are much more tolerant of temperature change than is often stated in aquarium publications. When a water change is made the temperature of new water should be within 5°F (2°C) of water in the aquarium.
If you have a balanced aquarium (nitrate does not accumulate over time), which is very rare, you cannot discount making water changes. Adding top off water over time can raise the TDS. Fish food also introduces additional TDS. For soft water aquarium, regular scheduled water changes should be done to keep the TDS low. An aquarium with a lot of live plants may require additional elements added on a schedule basis as plants will take up minerals as they grow.
Water changes are typically done by using a hose to drain water out of an aquarium into a bucket, drain, or garden. There are several aquarium manufacturers that make a hose/gravel vacuum water change kit that can be connected to a faucet. These devices make water changes very easy.
The gravel vacuum is a popular device that was developed to clean the gravel of aquariums that used under gravel (UG) filtration common in the 1970's and 1980's. Since UG filters use the gravel in the aquarium as a particulate filter medium the gravel vacuum had become a common tool for doing water changes. While gravel vacuuming an aquarium that uses UG filtration is essential, it can be detrimental to fish health on an aquarium that does not use this type of filtration.
The top ¼ inch (5 mm) of aquarium substrate is heavily colonized with aerobic bacteria that are responsible for keeping the ammonia and nitrite levels at 0 ppm. When a gravel vacuum is used to clean the substrate of an aquarium that does not use an UG filter it upsets the ecological layers of the bed, and removes heavily colonized aerobic bacteria layer from being in contact with the water column. Typically this will cause nitrite levels to reach toxic levels three to four days after the substrate was vacuumed.
On a modern aquarium that does don't use UG filtration, the substrate should not be disturbed when using a gravel vacuum to pull water out of the aquarium. The gravel vacuum can be used to pull detritus off the top of the substrate if there are any spots in the aquarium where it accumulates. It should never be used to do a deep substrate cleaning.
There is a long running fallacy in the aquarium hobby that states that if you do not gravel vacuum the substrate, hydrogen sulfide will build up in the aquarium and kill your fish. Hydrogen sulfide exist in all natural substrates in freshwater and saltwater. It occurs in anaerobic layers of substrate. It will not build up in sufficient quantities to cause harm in the aquarium, as other aerobic bacteria in the aquarium will process it, much in the same way ammonia and nitrite are processed. If gravel vacuuming was required, you would not see all those beautiful planted aquarium photos on the Internet, in books, and in magazines.
It is recommended that you change as much water as possible when conducting a water change. 100% water change is acceptable as long as the new water is close to the pH, temperature, and TDS as the water you are replacing. Typically, you will want to lower the water level to the point where your fish just have enough water to swim, then fill it up with de-chlorinated water. The more water you can change out the greater the reduction in nitrate. When changing water using the hose method, a dechlorinator can be added directly to the aquarium while filling up the tank. Dechlorinator immediately neutralizes chlorine in the water.
In cases where you might be keeping very large fish, like oscars (Astronotus ocellatus) where nitrate tends to accumulate very quickly, lower the tank water to the point where the fish just has enough water to keep wet, then fill the tank up to say 50% full, then drain again. Fill water at one end of the tank, and drain water out of the other end. This will help send nitrate rich water to the end that is draining the water. You can repeat the steps several times to try and get the nitrates as close to 0 ppm as possible.
How often you will need to do a water change on a aquarium will be directly related to how fast nitrate accumulates in the system. On an established aquarium, where standard fluorescent lights are used, and does not have live plants, weekly water changes may be required to keep nitrate levels below 100 ppm.
In aquariums that have high output lighting systems with fast growing live plants, and low fish population, it is possible that the nitrate will never accumulate. In this case you may only need to do a partial water change every three to six months and/or adjust the chemistry between water changes.
You should test your aquarium water after a water change for nitrate, then again a week after to see how fast nitrate accumulates. This will give you an idea of how often you will need to change the water in the aquarium.