First let me say that I am not in the pool business but a lady asked if I would take a minute and see if I could determine why her pool was going dry. It happens to be one of those preformed tubs like you would get at lowe's. After cleaning out the debris from the bottom I did find a crack in the bottom about two inches long. It appears something has pushed upwards from underneath and caused the crack. It was very easy to set the perimeter rocks off to the side and pick the "tub" straight up out of the hole. I now have it scrubbed down and cleaned. Next I am going to resolve the issue with what appears to be a root pushing up from the bottom. What can I use that is safe for fish and plants to seal the crack? Should I seal it from both sides or just one? Thanks for your help.