The biggest killer of fish in an aquarium is Ammonia. Ammonia is the bi-product of fish waste that builds up in the aquarium. The way the ammonia is removed from the aquarium is by either water change or by live bacteria that lives in your fish tank, amongst the gravel and the plants and also in the filter. The more bacteria that lives in your filter, the less chance you have of ammonia rising to dangerous levels.
To determine how much ammonia is in your aquarium, I use an Aquarium Pharmaceuticals ammonia test kit which comes with two bottles. Solution A and Solution B. To test put eight drops of Solution A and eight drops of Solution B into five ml of water and if it comes up yellow that means that there’s no ammonia in the water. If it comes up green, there is ammonia in the water. That will tell you that you need to do a water change or use some type of Ammonia Removing Product.
This Ammonia Test normally takes around about five minutes. Place the vial up against a white background and if the colour is yellow there is no ammonia in the aquarium water. If the test solution comes up green there is ammonia present. The ammonia can be very light green which would measure around .5 parts in a million or dark green which will be 4 parts in a million plus. This is extremely toxic and you will urgently require an ammonia solution and a 1/3 water change.