See this link and http://srac.tamu.edu/index.cfm?catid=25 then SRAC 0463 Ammonia in Fish Ponds

Dangerous short-term levels of

toxic un-ionized ammonia which

are capable of killing fish over a

few days start at about 0.6 mg/L

(ppm). Chronic exposure to toxic

un-ionized ammonia levels as low

as 0.06 mg/L (ppm) can cause gill

and kidney damage, reduction in

growth, possible brain malfunc-

tioning, and reduction in the oxygen-

carrying capacity of the fish.