Basic water chemistry

For beginner fish keepers, water chemistry can be a daunting subject. Some people may not even know they have to keep check of water quality! Either way, I want to try to make it as simple as possible. I always want my fish to be the happiest and healthiest they can be and I’m sure you want the same for your own. The best way to do that is to keep the different levels in your water at the best values for the fish in your tank.

First I want to talk about the nitrogen cycle. Nitrogen can be found in your tank in three forms, ammonium, nitrites, and nitrates.

Borrowed from
Borrowed from

Here’s a little drawing of how this cycle works:

That’s really all you need to know about how the cycle works. Next thing you need to know is numbers. There’s quite a few number values that are important in a freshwater aquarium: water hardness, carbonate hardness, pH, ammonia, nitrites, and nitrates. I’ll start with the last three because they are the ones that most often need to be controlled.

Ammonia: As shown in the cute little infographic, ammonia is created through decomposition of poop and waste materials. In a well established tank this number should be kept at a reasonable value. In my eyes 0ppm is the best, 0.5pmm is reasonable. When this level gets above 1ppm I start to worry about the health of my fish because ammonia is poisonous. When this level rises in an established tank you should look for dead fish and include lifting up decorations and vacuuming the places you usually can’t get to in your normal water changes.

Nitrites: Not as dangerous as ammonia, but still something that can damage your fish if the value gets too high. Nitrites are quick to develop into nitrates and in an established tank there is usually none present. In my opinion, if you’re waiting for your tank to go through its first nitrogen cycle, hold off on adding any fish until this number is reading 0ppm.

Nitrates: This is the most dangerous form of nitrogen in the tank. Ammonia and nitrites cycle into this product and it just builds up in your water over time. Live plants do consume some amounts of nitrates, but unless it’s a heavily planted tank, and I do mean heavy, you can’t really count on the plants to help you out. Just like ammonia, 0ppm is the best value, but it really never happens like that. Ideally 5-20ppm can be the value you maintain and there won’t be any health effects on your fish. Many people say fish can live healthily in up to 40ppm, but that really shouldn’t be constant. I start to worry at that level and do an immediate 25% water change.

The next four values are general hardness, water hardness, carbonate hardness and pH. The general hardness value is used to find your water hardness value, and your carbonate hardness can affect your pH.

General Hardness (GH): This refers to the dissolved minerals in your water. The higher the number, the harder your water is. The water in my area has a 120 hardness, it’s considered soft to slightly hard.

Carbonate Hardness (KH): This refers to carbonate concentration in the water. This value is much easier to raise than it is to lower. You can raise it by adding a source of calcium to your tank or lower it by using reverse osmosis water instead of regular tap water. My local water has a carbonate hardness of 80, this is the perfect level for all my fish, but I do have to add a calcium supplement for my nerite snails.

Water Hardness: This is the general hardness value measured in degrees. When you use freshwater test kits such as the API test kits, you won’t see this value, however many people come across it in their research about potential fish for their aquariums. The general hardness value in ppm is helpful for making changes to the levels because it’s much more accurate.

To find your water’s degree of hardness value, divide your GH value by 17.9.

A little chart I found from sets up these values quite perfectly:

Degrees of Water Hardness      mg/l or ppm Calcium Carbonate      General Description
0 – 30                                           0- 54                                          Very Soft
3 – 6                                             54-108                                                 Soft
6 – 12                                        108 – 216                                Slightly Hard
12 – 18                                     216 – 324                         Moderately Hard
18 – 30                                      324 – 540                                             Hard
30+                                                540+                                         Very Hard

There’s a super handy site,, that basically does all the math work for you regarding stocking your fish tank. This site tells you whether the fish you’ve selected are compatible behavior wise, or if their preferred water conditions are too different. It uses the degree of hardness value so the chart and division tip above will be helpful.

I hope this makes water chemistry somewhat easier for at least one person. I feel like I’ve done a pretty terrible job at explaining things after reading through it, but I really don’t want to get into anything technical. It’s just supposed to be the basic theory behind how things work in order to keep your fish healthy. Because healthy fish means happy fish, and happy fish means happy keepers!


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