Ammonia exists in two states when dissolved in water, and the ratio between those is governed by the pH of the water. Test kits typically measure total ammonia. Total ammonia is comprised of two components: un-ionized ammonia (NH3) and ionized ammonia (NH4+). Un-ionized ammonia is extremely toxic to fish whereas ionized ammonia is not. At lower pH levels, there is very little NH3 present in the water, so the fish are not as adversely affected. That's why overall, ammonia is more toxic to marine fish (higher pH). 7.8 is safer than 8.2 in terms of toxic ammonia present, but it can still be an issue with there is lots of total ammonia in the water, even down to a pH of 6.5
Jay