Pumps
What is the purpose of a pump in a fish tank? Well, you need to know a little bit of basic bio-chemistry if you want to understand why. If you don't want to know why, just take it from me that a pump is probably an essential piece of kit in your tropical fish tank. Do discuss whether or not you actually need one with your local tropical fish stockist - it may be for your tank that you have other pieces of kit (notably the filter system) which means that a pump is not an absolute necessity.
But anyway, back to a spot of 'O' Level biology / chemistry. Fish and their cohabitors in your tank are living beings, so naturally they produce waste material. This will consist of faecal material, which your filtration system will deal with and carbon dioxide (CO2), which is produced in the respiratory process (i.e. by breathing) - this is the waste material which your pump will help to deal with. Some of the CO2 will be used by the plants in your tank, however they won't use all of it and the surplus is released into the surrounding water. Your fish need to breath oxygen, which is dissolved in the water. At times, the level of oxygen can become too low; and that of CO2 too high for your fish.
At the water's surface, an exchange takes place between the atmosphere and the water. CO2 is released into the air and oxygen is taken in by the water. This happens naturally, but for there to be enough oxygen in the tank you either require a massive surface area, or some kind of turbulence at the surface. This is where your pump (or other means of surface agitation ) comes in.
