This is not complete true because fish has special channels/ion pumps in their gills that are able to active get rid of NH4 in the bloodstream – even against a concentration gradient, hence all the time have the NH4/NH3 concentration in the blood at a certain and safe level. There is nothing like long time build up NH3/NH4 toxicity IMO.
Sincerely Lasse
If this is correct I am badly informed and I have to review some articles about ammonia poisoning! Do you have references supporting this statement?
See Ammonia production, excretion, toxicity, and defense in fish: a review by Yuen K. , and Chew F.
The ammonia level in the water column may effect the presence of ammonia in the bloodstream. A high TAN ( Total Ammonia Nitrogen NH3 +NH4) level of +- 0.4mg/l may become a problem ( this is only +- 0,0045 mg/l NH3-N at 25° pH 8) When fish has to swim in such water for a certain period of time fish may show respiration problems and may not recover.
Many fishes are ammonotelic but some species can detoxify ammonia to glutamine or urea.
Certain fish species can accumulate high levels of ammonia in the brain or defense against ammonia toxicity by enhancing the effectiveness of ammonia excretion through active NH + 4 transport, manipulation of ambient pH, or reduction in ammonia permeability through the branchial and cutaneous epithelia (ref:YuenK. and ChewF2010)
There is no general rule but in closed marine aquaculture systems 0,4 ppm TAN is considered critical for growing fish. As in aquaculture fish is harvested after a relative short time these limits are set to avoid acute problems. Growing 80kg fish or shrimp/1000l from larvae till maturity needs a different approach and a very effective ammonium reduction system.
In an aquarium, a live support system, we try to keep a few fish alive as long as possible, a complete different approach, also for determining toxicity accumulation. As the bio-load is very low an acute problem may be solved but long term ammonia poisoning will take place without notice.
Ammonium does accumulate in tissue, brain cell membranes etc, and may kill a fish on the long term of slow poisoning., just that little bit to much.
Slow ammonia poisoning is about Ammonia accumulation between cell membranes in the cell wall , it may not to be removed by respiration as it may not enter the blood stream. It may accumulate from the day the fish was born, was captured and was prepared to be transported, during transport etc.. everywhere where it is confronted with free ammonia, The accumulation may continue until it becomes critical and the fish will die to young from old age symptoms. No need for high levels, contact time is enough.
This means an effective carrying capacity, the capacity to reduce ammonium, should be installed at all time. As most home reef aquaria have banned the bio the installed carrying capacity may be very limited.
Nitrate should not be considered a problem as the nitrogen content is easy to master.