~Nitrogen Cycle The ammonia can be in two forms. Toxic (to the fish) ammonia is in the form of NH3 and occurs when the ph is greater than 7.0. Nontoxic ammonia is NH4 and occurs when the ph is below 7.0. Ammonia is very toxic to fish. Ammonia is broken down into Nitrites (NO2) by special bacteria called "Nitrosomonas" bacteria. Then the Nitrites are broken down into "Nitrates" (NO3) by another special bacterium called "Nitrobacter bacteria". Nitrates are not harmful to fish unless they become high. The most effective way to keep nitrates low is through water changes. In a new aquarium very little of the nitrosomonas or nitrobacter bacteria are present, and need time to reproduce to sufficient levels to breakdown their respective nitrogen compounds. The bacteria attach to surfaces in the aquarium such as gravel (if you have it) or the material in the filter, such as sponges that have a large surface area for the bacteria to attach to. ~Homemade DIY Aquarium Filter This process requires the following items: Gravel, Filter floss or sponge, straws, tube, silicon tubing, air pump. It will take about 15 minutes to make this very efficient homemade filter.