"The only acceptable ammonia level in an aquarium is zero."
"...ammonia is lethal, and we should aim for zero levels at all times. Sublethal levels can also go unnoticed, causing tropical fish stress and making them more susceptible to disease."
"Even low concentrations of ammonia-N severely stress fish; this makes them vulnerable to diseases, thereby shorting their life span."
"Lethal ammonia concentrations at a pH of 6.5 are 0.73 ppm, while at pH 8.5, only 0.17 ppm is considered lethal to inhabitants."
"Ammonia can cause damage at levels of only 0.1 ppm (which is below the level detected by many kits!). There may be haemorrhaging and destruction of mucus membranes, the gills are particularly likely to be damaged, and may appear reddened."
"A nitrite level of only 0.1 ppm could prove harmful if exposure is prolonged."
"Left unchecked, ammonia is lethal to aquatic life at 1 part per million (ppm) and causes health problems as low as 0.25 ppm."
"Even if action is taken and the fish survives, a shortened lifespan and permanent damage may occur."
"Nitrite is dangerous to aquatic life at concentrations as low as 0.1 ppm, though this is somewhat species dependent and some species show greater tolerance than others. "
"Ammonia is highly toxic to fish even at very low concentrations; levels of 0.2 to 0.5 parts per million are enough to kill some aquarium species. The precise level of ammonia that will suffice to kill a fish depends on the temperature and pH of the water. Given the serious threat posed by ammonia to your fish, however, the best general rule is that it shouldn't be present at levels you can detect. If you can detect ammonia in the aquarium with a test kit, the level is probably too high."
"Rough guide to toxic levels of free ammonia:
0.020 to 0.049 (ppm) is considered 'tolerated' but will cause long term harm to its growth, immune system, health, etc. especially to eggs or very young animals.
0.050 to 0.199 (ppm) is perhaps tolerated for only a few days and is very harmful.
0.200 to 0.499 (ppm) is perhaps tolerated for a day or two and will probably kill.
0.500+ (ppm) is deadly and will probably kill within a day. "
Ammonia Toxicity Calculator
"Prolonged exposure to even moderate levels of ammonia (or nitrites) can result in fin deterioration and loss of color. This is noteworthy, as many aquarists will treat for fin rot or other diseases when the problem is either directly ammonia poisoning or a result of ammonia poisoning."
"It's also important to note that nitrite is toxic to fish at levels as low as 0.10 mg/L."
"How much ammonia is too much? The quick answer is: if a test kit is able to measure it, you've got too much (e.g., it's in enough concentrations to stress fish)."