How do you Build a Biofloc for a Fish Tank?

Biofloc is a method of fish and shrimp farming that uses a culture of heterotrophic bacteria to control water quality and recycle the nutrients already in the water. It lets aquaculture farms reduce, and in some cases stop, water exchange, and the microbial growth itself becomes extra feed for the stock.

Biofloc technology (BFT) is a low-cost way to control the culture environment. Heterotrophic bacteria take up toxic ammonia directly and turn the nitrogen into microbial protein that the fish or shrimp can eat, while nitrifying bacteria convert what remains to nitrite and then to less toxic nitrate. It is most useful where water exchange is very limited and stocking density is high, in ponds or tanks where quality, productivity and biosecurity all depend on keeping the water clean.

A biofloc system holds nutrients inside the pond instead of flushing them out, which lowers running costs. Because the flocs are eaten, it also improves how much of the feed ends up as growth.

Indoor tanks or raceways can also be used. Without natural sunlight, algae grow poorly or not at all, so the biofloc system relies entirely on bacteria. Big ponds need bottom drains to remove excess sludge regularly.

Once the pond or tank layout is chosen, plan the aeration system. Every biofloc system needs constant water movement to keep oxygen levels high and stop solids settling. Without movement, dead zones quickly lose oxygen and turn anaerobic, releasing large amounts of ammonia, hydrogen sulphide and methane. Any pond, tank or raceway needs a planned layout of aerators to prevent this. Ponds commonly use paddlewheel aerators.

Pre-seeding the culture water speeds up the start of a biofloc system and stabilises the pond. This can be done by inoculating the water with a commercial or home-made bacterial culture, or with water from a running biofloc unit.

Most species benefit from the better water quality, but choose species that can also eat and digest the flocs to get the extra protein. These are species that graze on or filter particles from the water, wholly or in part, such as tilapia and whiteleg shrimp.

Ammonia tends to spike early in the cycle, mainly from the nitrogen in feed. To prevent this, give the system a ready supply of carbohydrate (for example molasses or wheat flour) at the start. The carbon lets heterotrophic bacteria multiply and take up ammonia into new cells, which keeps the water clean.

Repeat this whenever ammonia threatens to rise later in production, especially at high stocking densities with heavy use of commercial feed.

With strong aeration, natural light (in most systems) and a steady carbon supply, the floc population grows quickly. Within a few weeks, floc volume rises from near zero to a few millilitres of settled floc per litre, depending on water temperature, available nutrients, sunlight and how much floc was seeded at the start. Floc volume is usually read by letting a 1-litre water sample settle in an Imhoff cone.

From then on, take water samples regularly to track water quality and the activity and density of the two kinds of biofloc. Outdoor bioflocs are mainly green algae and brown bacteria: the algae need sunlight to grow, while the bacteria mostly consume leftover feed, by-products and wastes.

Algae multiply faster at first, so a pond starts out green and then turns brown as bacterial colonies take over in the following weeks. As the stock grows and feed input rises, the system reaches a tipping point after which the water stays brown.

Once the system has turned brown, aeration must be increased sharply to match the high oxygen demand. A power cut at this stage can quickly kill the whole crop, because oxygen runs out and the decaying flocs and dying bacteria release ammonia in low-oxygen water. Keep the aeration running at all times, with a backup power supply.

Besides holding water quality at lower cost and without water exchange, a biofloc system improves growth rates and feed efficiency, which makes farming more profitable and sustainable.

Ocean Global, based in India, makes biofloc tanks from non-migratory PVC polymer for biofloc fish farming, along with PVC fish farming tanks.

Biofloc itself is a mass of suspended particles and microorganisms (bacteria, algae, fungi, small invertebrates and detritus) held together by the extracellular polymeric substances the microbes produce. It is a protein-rich live feed that forms when waste feed and excreta are converted into natural food in the culture system, helped by sunlight in outdoor ponds.