I'm a "newbie" to this hobby and this website. I setup a 300 gal poly livestock watering tank as a temporary pond. Had to help my son in rescuing abandoned koi left by renters. (long story...) At any rate, I've installed a 1inch bronze "through-hull" (used on boats) fitting at the bottom of the tank with a hand valve as a drain. I cannot get the rubber gaskets to seal properly. I have tightened, twisted, and used teflon tape, but to no-avail my efforts have been fruitless. I would like to use fast setting 3M 5200 sealant on the inside of the tank (full of water & 3 koi), but I'm afraid it might harm the fish. I know this sealant will cure the slight leak, even under water. I just don't know if it would harm the fish. Any information would be greatly appreciated! Thank you in-advance!!!