If it just appears to be a pin-hole type leak, you might be able to get away with using some putty type epoxy. It comes in a cylinder tube and you break off (and knead) a piece to activate it. No fumes and really easy to work with. I used it to fix a small leak in a DIY tank I bought a few years ago with good results.
Another option, and I'd recommend this for the exterior wood (on the backend) as well, would be to Drylok the whole thing. This is heavy duty paint used for waterproofing and has been used by quite a few folks on both concrete board and directly on plywood. It can be tinted to the desired color. I have yet to hear about a situation where the drylok itself failed in a DIY tank.
You could even use a combination of the two (epoxy putty over the leak and drylok teh whole thing).
I'd start with filling the thing with water and looking for leaks..