Assuming you can get at the tank's top I would get my hands on a hatch like this or similar
https://www.amazon.com/SEAFLO-Round-...6WGCTXR4BQ5Y2E
making sure I got one with a gasket, and put it in. I'd try to get it above the "normal" water line on the flat top, down down lower where it would see hydrostatic pressure...or at least as high as possible. I can't say with any certainty if these things would hold at or near the bottom of a tank.