RAS Ammonia or Nitrite Spike: How to Find the Cause and Protect the Stock

Short answer: an ammonia or nitrite rise in an operating RAS means the biofilter is getting more load than it can process, or it is processing less than it used to. Protect the stock first: cut or stop feeding, check oxygen and power, and add water exchange if you have the clean water and capacity. Then compare feed, biomass, biofilter condition, pH and alkalinity, temperature, oxygen in the biofilter, and any recent cleaning or treatment against the days before the rise. Fix the cause, not only the reading.

What do the terms mean?

• **TAN (total ammonia nitrogen):** ammonia plus ammonium. Fish and shrimp excrete it, mainly from digesting feed protein.
• **Nitrite:** the middle step when biofilter bacteria convert ammonia to nitrate. It is toxic at much lower levels than nitrate.
• **Nitrifying biofilter:** the media and bacteria that carry out that conversion. They need oxygen, alkalinity, a stable temperature and time to grow.

What should you do first when the reading rises?

• Re-test with a second kit or meter and a fresh sample, so you are not acting on a reagent or sampling error.
• Reduce or stop feeding. Feed is the main source of ammonia, so this is the quickest lever.
• Confirm dissolved oxygen in the culture tanks **and** at the biofilter outlet, and check pumps, blowers and power.
• If you have clean, temperature-matched make-up water, increase exchange while you find the cause.
• Involve your aquatic animal-health adviser if the animals look distressed. A water reading alone does not explain every symptom.

How do you find the cause?

Work through the system in the same order as the water flows. Most causes show up as a change in the week before the rise.

The FAO / Eurofish guide to recirculation aquaculture explains that the biofilter is sized from the feed load and that nitrification consumes alkalinity, which then has to be topped up (FAO). SRAC's overview of tank systems describes the same links between feed, oxygen, solids and water treatment (SRAC 451).

Is the problem nitrite and not ammonia?

If ammonia is under control but nitrite stays high, the second group of bacteria is lagging. That is common after a new start-up, a temperature drop, a disinfection event or a sudden feed increase. Nitrite-oxidising bacteria often recover more slowly, so expect the delay. For a new system, see why new RAS farms need biofilter start-up time.

When is it a design problem rather than an operating problem?

If the rise keeps returning at normal feed levels, check whether the system was ever sized for the biomass you now carry. Signs include a feed load above the design figure, biofilter media volume that was cut during value engineering, or solids removal that can't keep up. The RAS system design guide and sizing a RAS from feed load to oxygen demand explain the design basis to check against.

What records should you keep for the next time?

• Daily feed per tank and in total, with any change of feed type.
• TAN, nitrite, pH, alkalinity, temperature and oxygen, with time and sampling point.
• Every backwash, media clean, treatment, restocking and grading.
• Make-up water volume per day.

These records turn the next event into a one-hour check instead of a guess, and they are what an engineer or supplier will ask for.

Where FishMatch can help

If the investigation shows a capacity gap, use the RAS cost calculator to frame the upgrade, then send a reviewed RAS request with your feed load, species and existing equipment list. For other symptoms, start from the aquaculture problem solver.

Sources

• FAO / Eurofish, A Guide to Recirculation Aquaculture (Bregnballe, 2015): https://www.fao.org/3/i4626e/i4626e.pdf
• SRAC Publication 451, Recirculating Aquaculture Tank Production Systems: An Overview of Critical Considerations: https://srac.msstate.edu/pdfs/Fact%20Sheets/451%20Recirculating%20Aquaculture%20Tank%20Production%20Systems-%20An%20Overview%20of%20Critical%20Considerations.pdf
• SRAC Publication 4502, How to Start a Biofilter: https://www.srac.msstate.edu/pdfs/Fact%20Sheets/4502%20How%20to%20Start%20a%20Biofilter.pdf

Editorial note: written and reviewed by the FishMatch Group editorial team as general planning guidance. It is not engineering or veterinary advice.