This maybe a bit of a long shot, but have a read of this website.
The guy lives in Wisconsin which gets cold. He's some sort of engineer who does the whole aquaponics thing in his spare time so the problem he faces is the opposite of the one you face (he needs to keep his fish warm in winter).
Along with several articles about thermal mass, entropy etc etc, he's basically come up with a plan to keep his fish warm: convert old unused/dead chest freezers into fish-tanks. The idea being they're free or very cheap, already watertight and very well insulated. Because he eats the fish he coats them in something on the inside, but it's not too big of a stretch to maybe do the same thing to keep a body of water cold rather than warm.
Food for thought anyway. I appreciate a chest freezer doesn't look as pretty as a glass sided fish tank, but one stuck under your house, or a shady spot in general may work perfectly to prevent your water getting warm.
Since you only have small tanks you could maybe turn a bar fridge/freezer into a mini summer home for your guys?