Cecil, Hydrated Lime (Calcium Hydroxide) should be used with Alum to offset/balance the Alum's acidity, immediately. Ag Lime Calcium/Magnesium Carbonate/ is slow acting and for long term pH buffering/raising. Ag Lime will not react nearly fast enough with the Alum to prevent a pH drop/fish kill.
Makes sense Rex.
In my area our Ph is so high (8.4 to 8.5) and alkalinity so high (about 350 mg/l in my ponds) it isn't even necessary to add anything to counteract a PH drop after adding alum as it doesn't happen. When I did aquaponics I had a hell of a time dropping the ph and many times it would bounce back up.
I only use hydrated lime to kill off remaining fish in standing water in a drained pond. It literally burns up any frogs, tadpoles, or fish that get in the way. Pretty mean stuff!
