Living in a world full of microrganisims or 'germs' as some may call it we should take some time to reflect on the useful and 'good' bacterium out there, presenting the Nitrogen Cycle.
Basicly, as part of it's bodily functions fish excrete ammonia (NH3) as a byproduct of the respiratory and digestive systems. This ammonia is broken down by the bacterium nitrosomonas sp to nitrites. Nitrite (NO2) is then eaten by the Nitrobacter sp and converted into nitrates (NO3) that plants can absorb and use to grow.
(For those who actually read this text first, Gold Star!)