The Mystery of the dying fish 

Author: Dr. L.E. Freese

How many people have started an aquarium to only go through the heartache of losing the fish a few days or weeks after acquiring them?

 At the Rand Show recently, the most common answer to the question “ Do you have any fish at home?” was “Yes, but they all died!”

In the discussions that followed a general lack of understanding of the ecological system of an aquarium was revealed. The nitrogen cycle, which is the breakdown of toxic ammonia into relatively harmless nitrate, is one of the most significant cycles that occur in the aquarium.


Ammonia is found in the urine and faeces of the fish and it is also one of the chemicals that is formed by decaying plants and food that has not been eaten by the fish. (Therefore beware of over feeding.) The ammonia is broken down by down to nitrite by Nitrosomonas bacteria and Nitrobacter bacteria break the nitrite down to nitrate. The plants use nitrate to grow and the plants in turn are eaten by the fish. The nitrifying bacteria require oxygen, warmth, food (in the forms of ammonia and nitrite) and surfaces onto which they can adhere to, to survive.

In a new setup the tank is sterile and contains no bacteria and no ammonia. The bacterial colonies need time to multiply and this can take 2-6 weeks and in some cases up to 12 weeks. As the bacteria need ammonia to grow, usually 1 or 2 fish are introduced to the aquarium to provide the ammonia (the aquarium is never fully stocked at the beginning). Only when an aquarium is fully matured should you introduce new fish and this should also be done slowly and not all at once. If fish are added too quickly or too many at one time, an ammonia spike occurs 3-5 days later and the toxic levels of ammonia lead to the death of the fish. Doing partial water changes can reduce toxic levels of ammonia.

There are ways of speeding up the aging of the aquarium and those are the addition of gravel or filter material of an established tank or adding the commercially available dormant bacteria.


When cleaning the tank/aquarium, it should never be totally sterilized (boiling the stones and removing all the water) as this destroys the bacterial colonies. Nor should the filter and the aquarium décor (which includes the gravel) be cleaned at the same time. This would cause toxic levels of ammonia and nitrite to occur and you will have stressed unhealthy or even dead fish to contend with.

Cleaning should involve the cleaning of the front glass panel to remove the algae and dirt (remember that the utensils used should be free of chemicals such as soap). Removal of 25-50% of the water at least every 2 weeks is required to remove the build up of nitrates in the water. Lastly the filters should be washed in the water that has been removed from the tank in order for the bacteria not to be destroyed by the chlorine contained in tap water. When doing the water changes, do not forget to treat the water for the removal of chlorine and heavy metals.  

The fish hobby should be a relaxing and enjoyable one and if the proper procedures are followed when cleaning the tanks, it is almost hassle free.

August 2001