Waterproofing the tank the same way I've done my larger plywood builds. Probably overkill, but better to do it right.
Sealed with West System Epoxy:
First I taped the seams. I used the trick of laying the tank on it's edge, which let's the epoxy pool into seam. Really helps to fill any gaps, clear out bubbles and ensure that you build up a nice amount of material in the corners. Easier to do on this build since it's smaller and bit more manageable to move around.
After the corners were done, I also fiberglassed the bottom and walls up to where the waterline will be. Above that, I just did plain epoxy.
Total of 3-4 coats of epoxy