Guides

A recirculating aquaculture system, or RAS, grows fish in tanks and treats the same water again and again. Instead of drawing new water and letting it flow away, the farm cleans the water and sends it back to the fish.

UV and ozone are often compared as alternatives. They do different jobs. UV treats pathogens in the water flow. Ozone oxidises organics and colour. Many systems use both.

Two farms with the same tank volume can differ widely in cost. Volume is the easiest number to compare and the least useful. This guide explains which inputs set the budget, so you can judge any quote you receive.

A layout is only as good as the numbers behind it. Collect these before you ask any supplier for a design or a price. Missing a few is fine: send what you have and say what is unknown.

A RAS design is a chain of numbers. Start from how much fish you want to sell, and each step sets the next. Get the first numbers right and the equipment follows.

In a dense RAS, oxygen usually limits the farm before space does. Fish use most oxygen just after feeding, and they breathe out CO₂ that builds up in a closed loop. Both have to be managed.

Most RAS problems start with one stage sized on an average instead of a peak. Fish eat in bursts, solids arrive in waves and oxygen demand jumps after feeding. Size every stage for the worst hour of the day, and check that the stages agree with each other.

A new biofilter has media but no bacteria. Until nitrifying bacteria grow on it, the system cannot remove the ammonia fish produce. Stocking too early is one of the most common causes of losses in a new RAS.

A RAS tank does two jobs: it holds the fish and carries waste to the drain. Choosing a tank means choosing a shape, a material and a drain design that fit both your species and your building.

Filtration in a RAS removes three things: solids you can see, fine particles and dissolved organics you cannot, and ammonia. Each needs a different stage, and the order matters.