Excess ammonia level, usually more than 0.4ppm TOTAL ammonia. Toxic level depends mainly on pH, less on temperature. Cause can be dead fish in tank, malfunctioning biofilter, excess food, etc. While ammonia toxicity can be observed in both fresh and salt water, it is much more lethal in salt water as the higher pH level means the NH3 level which is major cause of death rises as a percentage of the total ammonia. The NH3 percentage is a gas which can penetrate the gill membranes of fish, causing rapid mortalities.
Find reason for excess level and correct. Fish will improve very fast on lowering ammonia levels.