by Shelli Wittig (Fishgal)

One of the most important aspects of successful fish keeping is good aquarium maintenance, including routine water changes.


If your aquarium looks beautiful and the water is crystal clear, everything is wonderful… right? Unfortunately not.

In an established aquarium, bacteria convert ammonia (produced mainly by decaying food and fish waste) to nitrite, and nitrite to nitrate (NO3). Nitrate is the end result of the Nitrogen Cycle, and while it is the least toxic form of nitrogen it does build up over time – to potentially dangerous levels if left unchecked. Changing a portion of the aquarium water on a regular basis is the easiest way to reduce it.

  • The buildup of nitrates (nitric acid) can reduce the alkalinity of your water, potentially resulting in a pH drop.
  • Phosphates, pheromones and other chemicals can build up between water changes.
  • Minerals and trace elements are depleted over time, which can result in a GH drop.
  • Poor water quality can lead to infection on a wounded fish.
  • Poor water quality puts stress on fish, which is often the root cause of fungus and parasite problems.

It has been said that dirty water holds less oxygen, although I do not have any scientific data to back this statement up. Without question, water changes supply clean oxygenated water to your fish – and that’s a good thing!

Frequency Edit

There is no standard answer to the question of how much water to exchange, or how often. It depends on several factors including the size of your tank, number of fish, feeding schedule and filtration system. Testing for nitrates is the most effective way to measure your water quality. As you begin to establish your maintenance schedule, perform a nitrate test before and after each water change, as well as once in between. As stated previously, NO3 builds up slowly and your goal is to keep it as low as possible; at the very least it should be less than 40ppm at all times. Once you determine a schedule that works for you, testing can be done less frequently to confirm your routine is adequate.

As a general guideline, you could start with a 20-30% water change once a week. Some people prefer to do a larger change less frequently, such as 40-50% every two weeks. I believe the latter is less desirable since the water chemistry changes more dramatically at once, putting more stress on the fish. Alternately, if your bio-load (ratio of fish to water volume) is low and you don’t overfeed, you may be able to do a smaller water change less frequently and still maintain good water quality.

Keep in mind as you determine the volume of water you intend to change, that a 100-gallon aquarium does not necessarily hold 100 gallons of water with substrate, rocks and equipment in the tank as well.

Further Reading Edit

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