The pH of your aquarium water is a measure of its acidity or alkalinity. The pH scale ranges from 0 to 14, with 7 being neutral. A pH below 7 is acidic, while a pH above 7 is alkaline. Most freshwater fish prefer a pH between 6.5 and 7.5.
Ammonia
Ammonia is a toxic gas that can be produced by the breakdown of organic matter in your aquarium. Even small amounts of ammonia can be harmful to fish, so it's important to keep the ammonia level in your aquarium as close to 0 ppm as possible.
Nitrite
Nitrite is another toxic nitrogen compound that can be produced by the breakdown of organic matter. Nitrite can also be harmful to fish, so it's important to keep the nitrite level in your aquarium as close to 0 ppm as possible.
Nitrate
Nitrate is the least harmful of the three nitrogen compounds, but it can still be harmful to fish if the levels get too high. Nitrate levels above 50 ppm can start to cause problems for some fish.
General Hardness (GH)
The GH of your aquarium water is a measure of the calcium and magnesium ions present in the water. GH is important for the growth and development of fish bones and scales. Most freshwater fish prefer a GH between 8 and 12 dGH.
Carbonate Hardness (KH)
The KH of your aquarium water is a measure of the bicarbonate ions present in the water. KH is important for buffering the pH of your aquarium water and preventing it from becoming too acidic. Most freshwater fish prefer a KH between 4 and 8 dKH.
In addition to these tests, you may also want to test for other parameters such as copper, iron, and phosphate. These parameters can be important for the health of your fish and plants, but they are not as critical as the parameters listed above.