Ammonia and Ammonium 

Ammonia is largely produced by the breakdown of organic matter i.e. fish wastes and decaying plants. It can be present as toxic ammonia (NH3), also described as 'Free Ammonia', and the relatively non-toxic form ammonium (NH4+).  

The actual proportion of each compound depends on temperature, salinity and most importantly, pH. A greater concentration of ammonia is present when the pH value is increased. Nearly 10 times more ammonia is present when the pH is 8.0 than when the pH is 7.0. At low pH levels, ammonia is more likely to appear as ammonium. Both types of ammonia are measured in test kits to arrive at the total concentration of ammonia in the water. Under ideal conditions, no detectable ammonia should be present in the aquarium. Levels over 0.02 mg per litre indicate a filtering deficiency. Certain species of fish tolerate ammonia better than others, but high levels of ammonia can create serious health problems and should be immediately corrected by a series of 30% water changes daily until levels become acceptable. Other remedies include the use of ammonia absorbers or the addition of a fully matured filter. Your local aquarium supplier may have these available for hire.

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