In an RAS, water is continuously reused, so any failure in solids capture or biological nitrification compounds rapidly with every turn of the system. Understanding how the mechanical stage protects the biological stage is the key to systematic RAS troubleshooting.
What Could Be Causing This?
- Solids overloading the biofilter (mechanical stage failure): If a rotary drum filter, settling basin, or bead filter lets fine organic solids pass through, fast-growing heterotrophic bacteria colonize the biofilter media, outcompeting slow-growing nitrifying bacteria for oxygen and space.
- Frequent drum filter backwashing or overflowing bypass: Caused by overfeeding (see why fish leave feed uneaten), dusty/crumbly feed pellets that break apart in pipes, clogged high-pressure backwash spray nozzles, or biofouled mesh panels.
- Low alkalinity or pH crash in the biofilter: Nitrifying bacteria require inorganic carbon (alkalinity) to function. If total alkalinity is depleted and pH drops, ammonia and nitrite conversion stalls abruptly. Review how to recognize ammonia problems in fish farming.
- Insufficient aeration or media channeling in moving-bed / fixed-bed biofilters: If air blowers weaken or air grids clog in a Moving Bed Biofilm Reactor (MBBR), plastic media stops tumbling evenly and clumps in anoxic dead zones. In pressurized bead or sand filters, infrequent backwashing causes media clumping ("mudballing") and water channeling.
- Chemical treatments or sudden salinity/temperature shocks: Adding uncalculated disinfectants, antibiotics, or salt directly through the main loop can strip or inhibit the nitrifying biofilm.
What Should You Check First?
- Test TAN, nitrite, pH, alkalinity, and dissolved oxygen before and after the biofilter: Healthy nitrification consumes oxygen and alkalinity; if DO exiting the biofilter is very low, the biofilter itself is oxygen-starved.
- Inspect the mechanical filter (drum screen, settling cone, or bead filter): Check backwash frequency, inspect screen panels for tears (which let solids bypass into the biofilter), and verify that all high-pressure backwash nozzles spray a clean fan pattern.
- Observe MBBR media movement or bead filter backwash water: Confirm that K1/plastic carrier media is rolling uniformly across the entire biofilter chamber without dead corners.
- Check system flow rate: If clogged filters have reduced circulation, review why water flow is weak in an aquaculture tank.
How to Tell the Possible Causes Apart
- Rising TAN and nitrite + low pH + low alkalinity: Alkalinity exhaustion stalling the nitrifying bacteria.
- Rising TAN and nitrite + normal alkalinity + thick brown/gray slime coating biofilter media: Mechanical filter failure allowing organic solids to smother the biofilter.
- TAN is low, but nitrite spikes persistently: Nitrobacter/Nitrospira (second-step nitrite-oxidizing bacteria) are slower to recover from temperature shifts, low DO, or chemical stress than ammonia-oxidizers.
What to Do Next
Immediately reduce or pause feeding—stopping feed input cuts new ammonia and solids production at the source within hours. Clean or backwash mechanical filters (using system water rather than chlorinated tap water on biological media so you do not kill beneficial bacteria), restore biofilter aeration, and replenish alkalinity gradually if tests confirm depletion.
When to Get Professional Help
Always lock out electrical power before servicing drum filter drive motors, high-pressure backwash pumps, or air blowers. If your RAS biofilter repeatedly falls behind at target stocking densities, consult an aquaculture systems engineer or water quality specialist to audit your daily feed load against your biofilter's specific surface area and oxygen transfer capacity.