The reason I'm fixated on a tube is that a muffin fan on the lid will dilute the whole amount of the viv's air. What I want is to spot-cool one 4 X4 or 5X5 inch area in one of the corners of the cool end, which now read 81.5 F. I'm aware that this cooler air in the corner will dissipate into the viv. But, the minute fan runs continuously.
My son is a computer systems engineer of some sort. He always has dozens of scrapped CPU's around in his work room, with their muffin fans attached. The muffin fans will work perfectly. They have various small diameters and various volts/mA. I will make some kind of blockage in the tube so only a small bit of room air makes it to the bottom. I know it will dilute the conditioned are inside the viv, but I'm OK with the added cost.
I guess I'll have to insulate the entire tube. Actually it will make the viv more efficient if I use one inch rigid foam on all sides except the front.
I'd better get this done soon, so y'all can see pics of it and a record of it's performance.
This certainly isn't the most important viv improvement in the world.