The Nitrogen Cycle

So I already dealt with an overview of the nitrogen cycle, the next step is to know of the different chemicals and bacteria involved in the two-stage process that is important in a closed system like an aquarium.

Ammonia

The fish excretes ammonia through the gill membranes (primarily the source of most ammonia in a fish tank) and additional levels are contributed by decomposition of waste materials (poop, leftovers, dead bacterial colonies for example) by heterotrophic bacteria. Ammonia exist in two forms (the more toxic ammonia [NH3, also known as free ammonia] and ammonium [NH4]) and most test kits will provide the sum of both forms (typically called total ammonia). At lower pH (acidic) range, more of the total ammonia exist in the less-toxic ammonium thus provides for a relative safer environment. Ammonia levels should always be maintained below 0.1 ppm (parts per million or 0.1 mg/L. In an established aquarium, there should be no ammonia as this is easily converted into nitrite (which should also be zero although it takes more time to convert into nitrate by nitrospira).

Ammonia is dangerous for your fish (it could kill and these are the things to watch out for: spasms, spinning in small circles and laying at the bottom of the tank. Other signs include increased mucus production, bleeding gills, darkening, gasping and increase respiration as ammonia destroys the gill membranes impeding gas exchange.

It is usually associated with the so-called 'new tank syndrome.' Ammonia is expected to rise and peak within 3 to 5 days of starting a new set up and should be zero at the minimum on the 10th day if the cycling is done properly.