This is a great application for shotcrete. Once drained, the existing surfaces should be cleaned by water blasting or sandblasting to provide a good bonding surface. The shotcrete can be submerged within a few hours or upon reaching the final set (hardened). One factor to be concerned about is the chemical reaction and alkalinity of the area around the shotcrete during the curing period. A good solution would be to submerge the pool for a period of at least a week, drain and waste the water, flush the shotcrete surfaces, refill the pool, and test the pH before reintroducing the fish. This should eliminate the danger of the alkalinity to the fish.