TAN and Ammonia

Total ammonia nitrogen, TAN, is the sum of the two forms in which ammonia exists in water: un-ionised ammonia, NH3, and the ammonium ion, NH4+. A standard test kit or probe reports TAN — the total — not the split between them.

That distinction is the entire subject. Un-ionised ammonia is the toxic form, because it is uncharged and crosses gill membranes readily. Ammonium is charged, crosses far less readily, and is markedly less harmful at the same concentration.

Ammonia is the waste that closes the loop. It is produced continuously, it is toxic at low concentrations, and in a recirculating system there is no dilution to carry it away. Managing it is the reason the biofilter exists and the reason the system has a biomass ceiling.

• Chronic exposure below acutely harmful levels still costs growth, feed conversion and disease resistance.
• The toxic fraction rises with pH, so a system that raises pH to help the biofilter simultaneously raises ammonia toxicity.
• Ammonia production tracks feeding, so the risk is highest at the moments of highest production intent.
• It is the parameter that most often reveals that a biofilter has been disturbed.

Fish digest dietary protein and excrete the resulting nitrogen, predominantly across the gills, as ammonia. A further contribution comes from the breakdown of uneaten feed and faeces in the water. Once in the water, ammonia partitions between NH3 and NH4+ according to an equilibrium governed by pH and temperature, and to a lesser extent by salinity.

The equilibrium responds immediately to a pH change. This is why a pH correction made to help one part of the system can produce an ammonia toxicity event without a single additional gram of nitrogen entering the water.

• TAN production is driven by feed, not by biomass. Two tanks holding the same weight of fish on different feeding regimes produce different loads.
• Un-ionised ammonia is the quantity of biological interest; TAN is the quantity measured. Converting between them requires pH and temperature.
• Effective solids removal reduces TAN production, because solids left in the loop mineralise into it.
• Ammonia removal capacity is biological and therefore lags load changes.
• Raising pH improves nitrification rate but increases un-ionised ammonia. The two effects pull in opposite directions and must be balanced deliberately.

• Base biofilter sizing on peak daily feed, at the least favourable temperature and pH combination the system will run at.
• Design solids removal as part of ammonia control, not as a separate objective.
• Provide alkalinity dosing so that pH can be held rather than allowed to drift, since a drifting pH moves the toxicity relationship as well as the nitrification rate.
• Provide sampling points that allow TAN to be measured entering and leaving the biofilter.
• Decide the exchange rate with ammonia excursion recovery in mind, not only nitrate control.

• Sample at the same point and the same time of day, at a fixed interval, so readings form a trend.
• Treat a rising TAN reading as a question about the biofilter, the alkalinity and the feed history before it is treated as a number to correct.
• Do not adjust pH upward during an ammonia excursion without recognising that it increases the toxic fraction immediately.
• Reduce feeding as a first response to a genuine excursion, because it is the only lever that acts on production rather than on removal.

• Calibrate pH probes on the same schedule as ammonia testing, since a pH error becomes a toxicity error.
• Check test reagents against their expiry and store them as specified; degraded reagents fail quietly.
• Cross-check the routine test method against a second method periodically.
• Maintain solids removal equipment, because its performance directly changes ammonia production.
• Confirm alkalinity dosing is functioning, not merely switched on.

• Recording TAN without pH and temperature, producing a log that cannot be interpreted afterwards.
• Comparing readings taken at different points in the loop as if they were the same measurement.
• Raising pH during an ammonia excursion and increasing the toxic fraction.
• Assuming a published nitrogen excretion coefficient applies to a different species, diet or temperature.
• Attributing rising ammonia to biofilter size when the cause is alkalinity depletion.
• Leaving uneaten feed in the system and treating the resulting ammonia as fish excretion.

Nitrogen retention in the fish is the upstream lever. Feed formulation, digestibility and feeding accuracy determine how much of the nitrogen bought as feed ends up as fish rather than as dissolved waste, and improving that ratio reduces the treatment duty rather than treating it more effectively.

The interaction between ammonia, nitrite and carbon dioxide in the blood of the fish is not a simple sum of independent stressors. Their combined physiological effect is why water quality is managed as a set of interacting parameters rather than as a list of individual limits.

Ammonia enters the water as a consequence of feeding, exists in a toxic and a much less toxic form, and moves between them with pH and temperature. Measure TAN, record the conditions with it, remove solids so less is produced, and protect the biofilter that removes what remains. An ammonia problem is almost always a biofilter, alkalinity or solids problem observed at the ammonia test.