I tried one to keep the pond cool a number of years ago. Set to turn on when the air temp was below 70°F, off when it was above that so I could keep the pond cool enough to keep trout alive.

Great idea, but while the thermostat worked as advertised, the water in the pond didn't read the instructions and it warmed up top to bottom above the 70°F mark. So, while in theory it seems like a good idea, I have yet to see hard data proving that it actually changes the water temp in the pond more than a few degrees from what the surface water temp is.

Somebody please prove me wrong with hard temp data.

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).