Recirculating aquaculture systems (RAS) have become more numerous and equally sophisticated as land-based aquaculture production has increased. Recirculating aquaculture systems represent a new way to farm fish. Instead of the traditional method of growing fish outdoors, this system rears fish at high densities, in indoor tanks with a ‘controlled’ environment.

Recirculating systems filter and clean the water for recycling through fish culture tanks. Water is typically recirculated when there is a specific need to minimize water replacement, to maintain water quality conditions which differ from the supply water, or to compensate for an insufficient water supply.

There are innumerable designs for recirculating systems and most will work effectively if they accomplish aeration, removal of particulate matter, biological filtration to remove waste ammonia and nitrite, and buffering of pH. These processes can be achieved by biofilters. These are living filters composed of a medium (corrugated plastic sheets, beads, or sand grains) upon which a film of bacteria grows.

The bacteria provide the waste treatment by removing pollutants. The two primary water pollutants that need to be removed are (1) fish waste (toxic ammonia compounds) excreted into the water and (2) uneaten fish feed particles. The biofilter is the site where beneficial bacteria remove (detoxify) fish excretory products, primarily ammonia.

Re-oxygenating the culture water as it returns to the fish tank is crucial. Oxygen is the first limiting factor in recirculating aquaculture systems and with less than the required levels most fish and other aquatic organisms will die in a very short period of time. It is also critical that biofilters have access to adequate oxygen.

Biofilters are homes to nitrifying bacteria, which are aerobic (use oxygen during respiration). Furthermore, nitrification, the conversion of ammonia to nontoxic nitrate by the bacteria, cannot occur without the presence of oxygen.

This system significantly reduces the amount of space and water needed to intensively produce seafood products. Filtration systems work continuously and keeping water clean. In this way, healthy environment is provided for fish.