Whether you keep freshwater fish or a micro reef system, you'll need to measure toxins and other essential elements in the aquarium. Below is a list of compounds which are tested for frequently.
Ammonia is the most troublesome and toxic of all.
In established tanks with proper filtration, ammonia should be undetectable. Ammonia is introduced into the tanks by fish urine, digestive waste and decay. Fresh and marine. When Ammonia is oxidized by nitrosomonas bacteria, it is converted into nitrite. The appearance of nitrite in the aquarium is a sure sign that your biological filtration is beginning to work.