While most filtration systems are able to remove ammonia and nitrite from aquarium water, nitrate begins to build up without frequent water changes. However, there is a way of removing nitrate without doing water changes, that is, denitrification.
Denitrification is a bacterial process which converts nitrate into harmless nitrogen gas. The catch is; the bacteria that are involved in this process are anaerobic (live in environments which lack oxygen or are ‘anoxic’). The bacteria that convert ammonia into nitrite and nitrite into nitrate, are aerobic (require oxygen). Therefore, in order to remove all of these compounds from your aquarium water, the filtration system needs to include both oxygen rich environments and oxygen depleted environments.
Oxygen depleted environments are most commonly created with deep media beds. These are difficult to create, as a fair amount of space is often required. The easiest way to create this type of environment is to layer different types of media, leaving minimal space between them. Start with mechanical filtration, to remove debris, then move to biological filtration (this should remove most of the oxygen) before moving back to fine mechanical filtration, such as filter floss. The final level of filtration should be deep enough to provide an anoxic environment. This is where ‘denitrification’ will take place. The problem with this however, is that if all the water flows through this area, flow will be drastically reduced and may become virtually no existent. Therefore, the trick is to provide two types of areas within your filtration system, one that will allow the easy flow of water and one that will not.