Biofloc Water Quality Management: C:N Ratio, Solids & Water Reuse

In a biofloc tank, the water is the filter. There is no sand bed, no separate biofilter doing the cleaning — the microbes suspended in the water do it, and they only do it well if you keep the water inside a fairly narrow window. Get the carbon-to-nitrogen balance, the oxygen and the solids right, and the same water runs for a full cycle or longer. Get them wrong, and the floc crashes, oxygen drops overnight, and you lose the batch.

This is the part of biofloc that the start-up guides skip. We have set up systems for farms in Ghana, Mozambique, Bangladesh and across Asia, and almost every problem call comes down to one of three things: the C:N ratio drifted, the aeration wasn’t enough, or the solids were left to build up. Here is how to manage each, with the numbers we actually use.

The C:N ratio: feeding the bacteria that clean your water

Biofloc water quality starts with the carbon-to-nitrogen (C:N) ratio. Fish and shrimp excrete nitrogen as ammonia; ammonia is toxic. To pull it out, you grow heterotrophic bacteria, and those bacteria need carbon to build cells. The working target is a C:N ratio of about 15–20:1 in the water.

You hit that ratio by adding a cheap carbon source on top of the feed. Practically:

• Carbon sources: molasses is the most common (cheap, fast-acting), followed by wheat flour, rice bran, cassava or sugar. Molasses is roughly 50% carbon, so it dosing is easy to estimate.
• How much: a common starting rule is to add carbon at about 50–60% of the daily feed weight when the feed is ~30–35% protein. If your feed is higher in protein, you add more carbon, because more protein means more nitrogen to balance.
• When: dose carbon shortly after feeding, when the ammonia pulse is highest. Split it across the day rather than dumping it once.

The signal that your C:N ratio is right is a stable, slightly sweet-smelling brown floc and ammonia and nitrite staying low (see below). If ammonia keeps climbing, you are under-dosing carbon. If the water goes thick and oxygen-hungry, you are over-dosing — or you simply have too much floc.

Dissolved oxygen: the number you cannot let slip

In a biofloc system, dissolved oxygen does two jobs at once: it keeps the fish alive, and it keeps the bacteria working. Floc bacteria are aerobic — they burn oxygen, and a dense floc can pull oxygen down faster than the fish do, especially at night when there is no algal photosynthesis.

The rule is simple: dissolved oxygen must stay above 4–5 mg/L, 24 hours a day. That means aeration runs continuously, and it has to do two things — add oxygen and keep the floc in suspension so it doesn’t settle and rot on the bottom.

Most farms cover this with a roots blower feeding a grid of nano aeration tubes on the tank floor. The fine bubbles oxygenate efficiently and the rising column lifts the solids. In larger or pond-based systems a paddle wheel aerator does the circulating work. For sensitive species or high density, some farms add a dissolved oxygen cone to inject pure oxygen on demand.

Two things are non-negotiable here. First, measure it — at minimum keep a DO meter, ideally a multi-parameter water tester that reads DO, pH, temperature and salinity together, because in biofloc the water is the equipment. Second, have backup power. A few hours of aeration failure is the single most common way to lose a biofloc crop, which is why a standby generator sits on most serious farms.

Floc density and solids: the part most farms get wrong

Floc is good — until there is too much of it. As a cycle runs, solids accumulate: more feed, more carbon, more bacterial biomass. Past a point, that thick floc starves the water of oxygen, clogs gills, and tips the system from clean to anaerobic.

You manage this by measuring, then removing. The standard tool is an Imhoff (settling) cone: take a litre of tank water, let it settle for 15–20 minutes, and read the settled-solids volume off the cone. The working range is about 200–500 mL/L. Below that the floc is still building; above it you have to take solids out.

How to remove solids:

• Settling chamber: the simplest method — divert flow to a quiet settling tank, let the heavy floc drop, decant the clean water back.
• Mechanical filtration: a rotary drum filter pulls out excess solids continuously without dumping water, which matters in a near-zero-exchange system.
• Reduce inputs: if solids are climbing fast, cut the carbon dose and review feeding rate before reaching for a filter.

The mistake we see most often is letting floc run past 500 mL/L because “more floc means more natural feed.” It doesn’t — past the window, the oxygen cost and gill stress wipe out any feed benefit.

Ammonia, nitrite and pH: reading the nitrogen cycle

Even with the C:N ratio dialled in, a new biofloc system goes through a maturation phase where the bacteria are still catching up. Watch three values:

• Total ammonia nitrogen (TAN): should trend toward <1 mg/L once floc is established. A spike means the heterotrophs can’t keep up — add carbon and, if needed, reduce feeding.
• Nitrite (NO₂): the dangerous middle step. Nitrite often peaks a couple of weeks in, as nitrifying bacteria establish more slowly than heterotrophs. Keep it under ~1 mg/L; high nitrite is a common cause of stress and mortality in young biofloc systems.
• pH and alkalinity: the bacterial activity consumes alkalinity and drives pH down over time. Keep pH around 7–8 and dose lime/bicarbonate when alkalinity falls, or nitrification stalls.

A good microbial community speeds all of this up. Many farms steer it with aquaculture probiotics — adding Bacillus and similar strains to dominate the floc with the bacteria you want, rather than leaving the community to chance.

Water reuse: how much, and for how long

The whole point of biofloc is near-zero water exchange. Done right, you exchange 80–90% less water than a flow-through pond, and you can carry the same water through a full grow-out cycle, topping up only for evaporation and solids removal.

In practice, “water reuse” in biofloc means three things:

1. Within a cycle — you don’t flush, you correct. Manage C:N, oxygen and solids and the water stays usable.
2. Solids draw-down, not water exchange — when you do remove something, you remove settled floc, not clean water. The water returns to the tank.
3. Between cycles — mature biofloc water carries an established microbial community. Many farms keep part of it as an inoculum to start the next batch faster, instead of starting cold every time.

The limit is salinity and accumulated solids. Each top-up and feed adds a little mineral load; over very long runs you eventually refresh part of the water. But the contrast with a conventional system is stark: where a flow-through farm discharges water daily, a well-run biofloc tank can go a whole cycle on essentially the same water.