I think he said it was gasket pipe and there is a 40' drop to the end of the drain outlet. That is near 20 psi. I'm guessing the gasket pipe is probably PVC sewer gasket pipe. Not sure what pressure it is rated for.
From what I gathered the 400 feet of 12" was basically drain pipe. Not sure if it would have been designed to hold pressure.
Replacing anything 12" pipe underwater would be not easy. I suspect loosing $3000 of fish and draining the pond would be cheaper and easier for the valve replacement.
Re-read his posts and the valve is indeed at a 40 ft depth. That is not quite 17.5 psi the valve is seeing or if capped at the outlet what the drain pipe would then see if the bottom of the drain is still at 40' (and not more). 17.5 psi against a 12" valve is some serious hydraulic pressure. 113 sq inches times 17.5 = 1977 pounds trying to push the valve off the end of a 12" pipe. Better have a good glue job and not be standing in front of it if it doesn't.
