Here's what I tried and didn't work.
A fountain/pond pump with hose going to a coil of hose in a fridge, hose dumps back into tank. Didn't cool the tank very well.
A fountain/pond pump with hose going to a coil of hose in a freezer, hose dumps back into tank. Got the tank to 62 on 85 degree days, but everything in the freezer melted.