use quick lime following alum, at about 1/2 the weight of alum, to maintain neutrality of the water.

example...400 # aluminum sulphate per acre followed by 200# hydrated lime to neutralize the added acidity of the alum. this prevents a drastic ph swing which can shock the fish. If you already have hard water and higher ph(8-8.5) adding the alum should not be detrimental. Just get some pool strips or a liquid pool test kit to keep an eye on it.