Drill another hole in the tank 3 or so inches up from the bottom and slightly to the right or left of the original hole.
Build a corner over flow (glass or acrylic) - Plumb this ala "herbie" drain - Lower hole is primary drain and the upper hole is the secondary / backup drain. Use egg crate to keep critters from going "over the wall"
This is a very quiet drain system. Here's pics of my plumbing on a 20H
Middle pipe is backup - slightly to the right and a little lower (with ball valve) is the primary
here's a closer pic of the egg crate overflow guard
