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Water Chemistry: What about it and why should we worry?

by Tim The chemistry of the water in a fish tank is very important. A fish tank is a closed environment, where everything that goes into it affects the chemistry in one way or another.

Imagine you are sealed into a room with no toilet, no windows and fed only when the keeper saw fit. What would the air be like in a week? This is essentially what happens in a fish tank. The fish are trapped in an environment that you control. They get fed when you give them food, the water gets changed when you remember to do it. Filters help a lot, by controlling the levels of toxins, removing large bits of debris and un-eaten food and aerating the water, either by disturbing the surface or by adding air internally in the filter. You can add air stones, plants and ornaments, but the fish are still trapped. The décor makes it a little more comfortable for the fish, but they are still trapped.

So, where does the water chemistry come into the frame? Without knowing what the conditions are inside the tank, you cannot tell whether the fish are comfortable or not. If you just wait till something happens, it’s usually too late, and you lose the fish. Testing the water is essential for healthy fish. Knowing what to look for is also essential, but this is something that you learn as time goes by.

Also, some fish can only tolerate certain conditions. If you don’t know what your water chemistry is, how do you know if they are going to survive?

What do we test for?

Ammonia: The levels of ammonia in the tank are critical. As little as 1PPM will kill.

Nitrite: Although not as toxic as ammonia, it is still lethal in sufficient quantities.

Nitrate: This is not nearly as dangerous to the fish and is tested to find out just how effective the filtration is and when we need to make water changes. Nitrates are normally present in a cycled tank, and should be kept at around 10PPM. This level is perfectly harmless. If the level rises to 50PPM you may start to see signs of distress and the fish’s immune system is weakened, leaving them more susceptible to disease. When it gets as high as 100PPM, the fish will be severely stressed and will start to show signs of infections and other problems.

pH: This is a term used for acidity of alkalinity of water. Most fish can survive in a wide range of pH, but some, especially African Cichlids need a much more specific pH.

GH: Is the general hardness of the water in your domestic supply and the tank. It refers mainly to the amount of calcium and magnesium in the water.

KH: This is the term used for carbonates and bi-carbonates, and this refers to the “Buffering Ability” of the water, as in how easy or difficult it is to change the Ph value of the water.

How do I test the water?

Test kits come in various forms, each type covers different aspects and the accuracy of the kit depends very much on the cost. Below are descriptions of two of the most popular.

Test Strips: Simple and effective, but by no means accurate. These can be used to test the pH, Gh, Kh, Nitrites and Nitrates in the water. They are only good as a guide, not being accurate enough to identify problems, but good enough to suggest that there may be something going wrong. I use them a lot on my tanks, simply because they are relatively cheap and as all my tanks are stable, it’s just a way of being sure.

Liquid test kits: Much more accurate, but more difficult to use. These will give much more precise readings on individual toxins, such as ammonia and nitrites. These should be used whenever you suspect that you have a problem, or when starting a tank, during the nitrogen cycle. They are more expensive, so one tends to be careful with the use of them.

Do I need to change any of these?

Any signs of ammonia is dangerous and should be addressed immediately by water changes and reducing the load on the water, by removing fish etc., find the cause of the increase. This could be due to adding new fish, blocked filter, overstocking etc. Mostly the cause is the filter, so checking your filter and cleaning it if necessary in old tank water may well rectify the problem.

Nitrites are similar, and should be addressed in the same manner as above.

Nitrates need to be held below 20 to be sure that your fish are healthy. This is done by regular water changes according to the readings. If your tank is overstocked, then more water changes will be necessary, if you have plants in the tank, this might reduce the need for changes a little, but they will still need to be done.

pH is something that we are pretty well stuck with, but it helps to know just what pH your water system has. It can be altered, but is very difficult to do, and has to be continually checked and re-adjusted during water changes. If possible, it’s best left alone.

GH and KH can be altered relatively easily by proprietary products, distilled water, rain water and the like. Personally, I have not found a need for this yet!

Other useful and interesting web resources:

Questions, answers and some examples

On March 20th 2011 we merged all water-related answers from with this article due to offering all related information at one place. If you'd like to ask and get answer, use a form at the bottom of this page, please. Some of the following questions may sound a little unrelated to water chemistry, but they're all questions that real people asked when visiting our website - and we answered them!

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