The biggest killer of aquarium fish is ammonia. It is the bi-product of fish waste that builds up in the aquarium. To remove it from the aquarium, you will change its water or have live bacteria living in your fish tank. The bacteria should stay amongst the gravel, the plants, and in the filter—the more bacteria that live in your filter, the less chance of ammonia rising to dangerous levels.
Use an Aquarium Pharmaceuticals ammonia test kit to determine how much ammonia is in your aquarium. The test takes around five minutes and comes with two bottles, Solution A and Solution B. To test, put eight drops each of Solution A and B into five ml of water. Place the vial up against a white background, and if the colour turns yellow, there is no ammonia in the aquarium water. However, if the test solution comes up green, ammonia is present, and you need to do a water change or use some Ammonia Removing Product.
The ammonia can be very light green which would measure around .5 parts in a million. If it is dark green, the concentration will be four or more parts in a million. The latter is toxic, and you will need an ammonia solution and a 1/3 water change.