There are good bacteria and bad bacteria.  Aquariums use good bacteria to eat toxic fish waste: creating a biological filter.                                  

The rocks at the base of the aquarium will be home to these good bacteria.

The bacteria need to grow into a good sized population, become ‘established’, so that they can keep the  water clean of fish waste.

Filter bacteria need food (fish waste) so that they can multiply. To do their job, the bacteria also need a constant supply of oxygenated water. This is provided by the air pump which must be left running 24 hours a day.

When the fish goes to the toilet ammonia is produced. Ammonia is deadly to fish. The bacteria convert the toxic ammonia into a safer substance called nitrate. For every small goldfish added it takes around four weeks for enough bacteria to multiply for that fish. This is why fish must be added at a rate of one every four weeks.

The right way: one small goldfish introduced

When the new fish goes to the toilet filter bacteria start to multiply to catch up with the amount of work they have to do. The fish should be fed very sparingly. After about 28 days there should be just enough bacteria to cope with all the waste from that one fish. The water quality should then be good and the aquarium is ready for one new fish. The process will happen with every new fish.

The wrong way: more than one goldfish introduced

If more than one goldfish is added at one time then there will suddenly be too much waste in the water.  Filter bacteria cannot multiply quickly enough and so struggle to clear up all he waste. The extra waste will build up and pollute the water. The polluted water can be clear, cloudy or eventually brown or green with algae growth.This is very unpleasant for your fish, breathing can become difficult and they may die.