Rob -
One other thought regarding the max amount for a water change.
IMO, if it's an emergency, a 100% water change is a posibility if it is the only option. My friend had a Rio pump leak oil into his tank. The water was extremely polluted, and he ended up replacing almost all of the water. While some corals died, many survived, as did all the fish. So I guess if it's a drastic enough emergency, I would have no problem with water changes greater than 40% - 50%.
