Pete, I used to work through central La. & my dad lives in Baton Rouge, so between that and our soggy Mid-Atlantic summers here I can identify with the shovel smack in the face humidity. I've got a lot of the same issue.
A lot of folks on other forums suggest air exchange as a solution to dehumidification. Well that might help if my inside R/H is 75% and it's 30% outside but that only happens during the worst of winter here. In the land of Spanish Moss and Magnolias it's even worse. My first time there I had to run my rental car air conditioner for 15 minutes at the airport before I could clear enough interior humidity just to see out of the windshield. Then the outdoor humidity would condense and I had to keep running my wipers as though it was raining.
Since your temperature differential between tank and room is usually going to be pretty close I'd guess that your floor is going to be the first potential complication for excess humidity. If the slab isn't insulated and the soil is sandy it will work like a giant heat sink and the floor's cool surface could become a huge condensing plate if your R/H gets too high. Since dragging high R/H air from outside in isn't likely to do much most of the year you're kind of stuck with collecting the interior moisture and getting rid of it.
If you try to just circulate high R/H air all that you do is help delay the condensation event until the air is more heavily saturated (yes I have tried it and yes I have made it rain indoors). Insulating the tank and keeping a lid on all of your open vessels will certainly go a long way toward reducing all of that. I've had to go back and make new lids and infills for most of my present tanks to accomplish just that and it's made a huge difference. That and getting rid of or sealing up anything with a biowheel on it (great little evaporative humidifiers they are).
I'd also suggest that you maybe see about throwing a coat of vapor barrier paint on the wall sufaces. Zinzer makes one that's alcohol based and dries in 15-20 minutes but it's hell to use (horrible fumes -gotta have a vapor mask). The good part though is that it will prevent any moisture from penetrating the surface of your drywall and allowing mold to set up shop. It's a good back up for times when the A/C or dehumidifier shut down or Mr. Murphy throws ya some other curve.
Mold needs warmth, humidity over about 40% and an organic food source (anything containing carbon...like exposed wood surfaces, drywall paper, insulation backing, etc.) As long as you keep everything sealed up and the R/H low you won't have a problem. Actually given that you only have you filtration/circulation system and tank top to seal it'll probably be pretty easy for you, in spite of the size of Lake Nola.
