If you don't change the aquarium water for a long time what will happen to the fish in it why?

Answer 1

Fish create bi products in just the same way as all other forms of life do. We all breath in air. We use some of the oxygen in the air and breath out carbon dioxide and other gases that we can't use. We also eat and drink and pass out urine and faeces. In the case of water living creatures, the excess Carbon dioxide gas goes out of the water and Oxygen is taken in (transpiration) at the surface of the water (the meniscus), so the ratio of oxygen to the other gasses in the water remains the same. In nature there are naturally occurring bacteria that break down toxic urine and faeces, and these toxins are made benign. The most destructive of the bi-products of life in water is the production of Ammonia. This is deadly poisonous and will kill almost anything. In a tank, a filter is used to do what nature does. One species of bacteria in the filter medium converts the toxic Ammonia into not as toxic Nitrite and then another lot of bacteria converts the Nitrite into relatively harmless Nitrate that can be used up by plant life (algae) and other things. Now I have explained what happens in nature you can see that if the tank is filtered well and not overstocked, there will be a steady increase of Nitrate in the water but the fish may survive OK. The reason for regular water changes is to reduce the buildup of Nitrate. If the tank is large enough and nature has taken its course there will be heaps of Ammonia converting bacteria, heaps of Nitrite converting bacteria and heaps of algae using up the Nitrate keeping the water balanced. If the tank is not large enough and the tank is overstocked, nature will be unable to cope and the fish will die before a natural balance can occur.