DrDon,
Not 100% sure, but I think it's highly unlikely that gypsum will harm fish.
I over applied by a huge amount in my minnow pond just to see how the minnows would react. Even with 10x more than I needed, the fathead and gambusia never even reacted. I previously did the same experiment with Alum. The fish kept eating and behaving normally. It appears to me that fish have a huge tolerance for mineral load and pH swings. It may affect spawning, of course, and some species may be a bit more sensitive than others.
