Biofilter oxygen and ammonia in RAS: the nitrification link
In a recirculating system, the biofilter clears ammonia only as fast as its bacteria can breathe. Nitrification (the bacteria converting ammonia to nitrate) is oxygen-hungry, taking several times the oxygen of the ammonia it processes. When dissolved oxygen runs low, the biofilter falls behind and ammonia climbs. Oxygen does not remove ammonia; it lets the bacteria that do keep up.
What’s actually happening in your water
A recirculating aquaculture system (RAS, where the water is cleaned and reused rather than flowing through once) leans on its biofilter to keep the water safe. The biofilter is a bed of media that houses nitrifying bacteria, which carry out nitrification: converting ammonia, which fish excrete and which is toxic, first to nitrite and then to far less harmful nitrate. It is the core of how a RAS stays livable between water changes.
That conversion is aerobic, and it is oxygen-hungry. Oxidizing a milligram of ammonia nitrogen takes on the order of 4.6 milligrams of oxygen for the reaction, and once the bacteria’s own respiration is counted, the total demand can approach ten milligrams of oxygen per milligram of ammonia. If dissolved oxygen (the oxygen dissolved in the water) at the biofilter runs low, the reaction slows, and ammonia the filter would have cleared builds up in the system.
There is a competition underneath this. Nitrifiers are slow growers, and when the organic load is high, faster heterotrophic bacteria consume the oxygen and colonize the surfaces first, starving the nitrifiers. So a biofilter can look established and still fall behind, because the oxygen its nitrifiers need is going elsewhere.
Why the usual fixes don’t hold
The reflex is to add biofilter media or scrub the numbers with a water exchange. More media does nothing if the media you have is oxygen-starved, and a big water change dilutes the ammonia while defeating the point of a recirculating system, spending the heated, conditioned water the design exists to keep.
Sharing one oxygen supply across the culture tanks and the biofilter is the other trap. When the tanks take priority, the biofilter runs on what is left, and its nitrifiers are exactly the community that suffers when oxygen is short.
How restoration works here
Nanobubbles stay suspended and give their oxygen up in the water rather than the air, so oxygen reaches the biofilter and the tanks efficiently and continuously. Held high enough, it supplies both the heterotrophs and the slower nitrifiers, so the nitrifying community works at its full rate and the filter keeps pace with the ammonia load.
To be exact about the boundary: this supplies the oxygen nitrification demands, and the bacteria do the removal. Oxygen is not ammonia removal. We install the system, and Stewardship logs dissolved oxygen at the biofilter and the tanks alongside the ammonia and nitrite you already track. What we measure and how is published, so the support shows up as a number you can check.
What to expect, and when
Weeks 1-2
We baseline dissolved oxygen where it matters: into and out of the biofilter and across the culture tanks, so any oxygen shortfall at the filter is a measured number.
Weeks 2-6
With oxygen held steady to the biofilter, the nitrifying bacteria work at their rate rather than a starved one. Where oxygen was the limit, ammonia and nitrite processing steadies on your own logs.
Ongoing
The record is dissolved oxygen at the biofilter and the culture tanks alongside the ammonia and nitrite you already track, so the biology has the oxygen its load demands as stocking changes.
The record
We don't have a published case file for this problem yet. Every Alchemal installation is instrumented from day one, so the first case files are being measured now, and until one is ready, our methodology shows exactly what we record and how we report it.
When this isn't the right fix
- If your biofilter is undersized, immature, or recently disinfected, oxygen is not the limit; capacity or colonization is. A new or damaged biofilter needs time and surface area, not more oxygen, and the assessment will say which you are facing.
- Ammonia also climbs from overfeeding, a die-off in the tank, or pH and temperature outside the range the nitrifiers can work in. Oxygen supports the bacteria; it does not correct a feed rate or a pH problem, so those get checked first.
- This is oxygen support for the biology, not ammonia removal. Nanobubble oxygenation supplies the oxygen nitrification demands; it does not strip ammonia from the water, and we will not describe it as if it did.
Questions people ask
Why does my biofilter fall behind on ammonia?
Usually because the nitrifying bacteria are short of oxygen. They are slow-growing and easily outcompeted: when organic matter is high, faster heterotrophic bacteria consume the available oxygen first and crowd the surfaces, so the nitrifiers slow down. As they slow, ammonia they would have processed accumulates in the water.
How much oxygen does nitrification actually need?
A lot relative to the ammonia involved. Oxidizing one milligram of ammonia nitrogen consumes roughly 4.6 milligrams of oxygen for the reaction itself, and once the bacteria's own respiration is included the total demand can approach ten milligrams of oxygen per milligram of ammonia. That demand has to be met, or the reaction slows.
Will adding oxygen lower my ammonia?
Only indirectly, and it is worth being exact about how. Oxygen does not remove ammonia. What it does is let the nitrifying bacteria process ammonia at their full rate instead of a starved one, so where oxygen was the bottleneck, the biofilter keeps pace with the load and ammonia stops climbing. The bacteria do the removal; oxygen lets them.
Is oxygen competition in a biofilter a real problem?
Yes. In a heavily loaded system, heterotrophic bacteria growing on organic matter consume oxygen quickly and can leave the slower nitrifiers short, which drops ammonia-processing rates even when the filter looks healthy. Holding dissolved oxygen high enough that both communities are supplied is what keeps nitrification running.
How is nanobubble oxygenation useful in a recirculating system?
It puts oxygen into the water efficiently and continuously, where it can reach the biofilter and the culture tanks without the losses of surface aeration. Because nanobubbles stay suspended and release oxygen through the water, a steady supply reaches the nitrifiers, and Stewardship measures the dissolved oxygen so the support is a number, not a claim.
Tell us what your water is doing.
A specialist reads your description and replies with a plain answer: what it usually means and what we would measure first.