Sorry, no advice, but it sounds like you are really trying to do the best for your tort, who is gorgeous, btw!
I think it is a great set up. I hope you get that mold problem licked.
I covered all interior surfaces in a silicone caulk, let it cure for a couple weeks and haven't had any mildew problems for the last 4 month's. I tried a sealant and it jut didn't work mold and mildew wise... the caulk seems to have worked wonders
Interesting. One of the tanks is OSB marine grade, and the other seems to have some kind of treatment on it (this came with the tortoise). Both tanks are currently covered inside in a waterproof tarp. Though not a big problem, the mould (when it does appear) is mostly on the glass front panels!
I have pondliner coming in the post, and am going see how it gets on. If it doesn't improve, next spring I am going to build a new enclosure, and seal it all up with the appropriate product (something like epoxy resin).