The Importance of Regular Water Changes
By: Craig Morrison


The solution to pollution is dilution1 is a very important concept that beginning fish keepers should learn. The fish we keep are producers of waste, as is the food we feed them. If we want to keep our fish healthy and thriving, regular, partial water changes are a must. I spend a lot of time reading many message boards relating to the proper care and maintenance of keeping fish.

The most often over-looked subject is water changes. Beginner’s simply don’t understand the necessity of keeping the water in their fish tanks clean. What I hope to do in this short article is provide a little insight into why it is important to do partial water changes.
Test Kits

When should water changes be performed?

The answer is, when the levels of toxic chemicals in your tank start to accumulate and reach levels that can harm your fish.

All aquarists should keep a basic set of test kits. Which would consist of kits for:

  • ammonia
  • nitrites (useful during tank cycling)
  • nitrates
  • pH

Water Changes During Tank “Cycling”

When a new aquarium is set up it has to go through a process called “cycling”. The Nitrification Cycle, which is the process of converting ammonia->nitrites->nitrates is one of the most important aspects involved in the successful keeping of fish.


Respiratory cycle
photo from Rolf C Hagen Inc.

If your goal is faster cycling or larger bioloads, I’m afraid you may be disappointed. Fishless cycling is not designed for this, and rarely can either of these be realistically achieved any better than traditional cycling. Both require a certain amount of time. However, there is one main difference. Here is where I get into my philosophy, so bear with me, but I really think you need to understand this to justify your use of this method. Fish have rights. Just like any living animal which is in our care, that animal was at one point, whether domesticated or not, taken from its natural environment. At that point, we took on the responsibility as aquarists to provide a suitable and favorable environment for our fish. One way to do this is by utilizing fishless cycling.

Performing water changes during the initial cycling of your tank WILL increase the time it takes for the process to come full circle. In my eyes it is more important to reduce the stress on the fish then it is to have a speedy cycle. You can skip all this and not have to worry about stressing any fish by doing a fish-less cycle.2

As soon as fish are introduced into a new tank, the process begins. Fish produce waste, in the form of ammonia in liquid waste and solid wastes which eventually break down. Partial water changes should be performed to keep the level of ammonia below the toxic level. I’m not advocating large water changes, but rather small water changes to keep ammonia levels in check so that the levels stay below toxic levels when using fish to cycle a new tank. Buy an ammonia test kit and pH kit, use them to tell you when the levels are getting dangerously high.

Note: Any testable level of ammonia is potentially toxic. This depends on the pH of your water. See How much ammonia? for more information and a handy chart.

Once nitrites are being produced and ammonia levels fall, you need to have a nitrite test kit to monitor nitrite levels in your tank. When the level of nitrites start to rise above 1-2 ppm you should perform a partial water change to keep the nitrite level down. How much water to change is totally dependent on the size of your tank, so you will have to use some common sense and your test kit to give you an indication of what is right for your situation.The last stage of the nitrification cycle in your tank is when nitrates are being produced and nitrite levels fall. This is the stage where your tank is now “cycled”.

After the Tank is Cycled

Over time fish wastes and other dissolved organic compounds (DOCs) begin to build up in the tank water. The ONLY solution and remedy for removing them is to do regluar, partial water changes. How much water to change and how often is, again, dependent on tank size and bio-load (number of fish, filtration). This is where a nitrate test kit comes into play.

Some people recommend 15-20% every 7-10 days. Which is a good general rule of thumb. However, regular testing of the nitrate levels in your tank is a better indicator since your tank will not necessarily be the same as everyone else’s.

In a perfect world nitrate levels should be as near zero as possible. In our enclosed eco-systems, this is nearly impossible to acheive. So we need to resort to manually changing water. Keeping nitrate levels below 40-50 ppm will do a good job of keeping your fish healthy. Ideally, keeping nitrates lower would be better.

In Closing

If we keep the water clean in our aquariums our fish will amuse, amaze and delight us with their antics. Clean water means a healthier environment for our fish which will reduce the possibility of disease and make for a more comfortable home for them.


[1] Quote stolen directly from the FAQ at
[2] Fish-less cycling Fishless Cycling:

Most of the information here was obtained by reading the entire FAQ at The Krib.

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