Way overdue for an update on this build!
Back on 10-13-17 I built the in-tank air circulation system based off a concept that my friend Dr. Adeljean Ho showed me. He had used corrugated plastic (obtainable at the big box hardware stores) to create a ductwork system that would be hidden behind the foam wall of his vivarium.
In Ho's setup, he placed supports for the fan to sit horizontally near the top of the riser, the actual ductwork somewhat "open" at the top, sealed by the cover glass. In this orientation, the fan could be easily removed for servicing/replacement.
I really liked his concept, but disliked the amount of space that a fan, oriented horizontally, would require. This is actually my second build for this concept; my first was done in my daughter's 10-gallon with a small 40 mm fan. I found that while the fan is certainly moving air, it's doing nothing for clearing up condensation in her tank. For my wife's classroom vivarium, I wasn't going to take any chances.
So this build uses an 1IP 56 rated fan; I wanted to go higher, but many factors prohibited it on this build. Nevertheless, I looked at IP56 given the dustproof and splash resistance...there's no telling what a 3rd grader may do with a handheld mister.
I'll say now that I tried two PWM (4-pin) Rosewill 120 mm versions, using the Perry Lee 4 channel fan controller - https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
- they fried in very short order despite supposedly being compatible. When I received a regular, 3-pin version of this fan, it ran FINE and consistent for weeks on the controller. Rosewill and New Egg both were more than happy to just keep sending out fans despite the fact that they were failing...in the end I now have some other fans I didn't even want, 2 semi-dead ones...it should not have been this complicated (I'm not exactly thrilled with my first fan / power supply / control setup either...this is, to date, my biggest gripe with the vivarium hobby).
Anyway, I went with the 120 mm fan and controller on the premise that if it's too much, it can always be dialed back. I can't make my 40 mm fan push any more air unless I swap out the fan itself.
Getting back to the design, I came up with a slimmer profile and a top-loading concept that wouldn't require several square inches of space to be taken up by the ductwork behind the foam background. I was concerned it might restrict flow or something, but that doesn't *seem* to be the case, although the fine-mesh fiberglass screen certainly does slow the fans down a bit (another reason to go bigger than you think you'll need).
A razor blade, black duck tape, and that's about all it took to build out this air circulation system. It pulls from the bottom, exits at the top. The photos should be pretty self-explanatory.