Here is excellent advise from Patandchickens, copied from another thread (winter topic)....
Heated waterers: You have basically 4 options. Heated dog water bowls and the like; plastic waterers with a built-in heating element in the base (I just bought a 3 gallon one from McMurray, will see how it works this winter); heated metal bases that you set metal (or possibly *certain* plastic) waterers on top of; or build-your-own versions of the latter, of which there are various designs.
I would suggest that if you never experience more than like 12 hrs of freezing temperatures at a time (or it can be more, on occasion, if you don't mind manually hauling water) then there is no reason to buy a heated waterer. Even Way Up North, you can sort of get away without one even in extremely cold weather by just putting out fortex tubs of water several times a day. However personally I think it is not only easier for the chickenkeeper but kinder to the chickens, in very cold climates, to make sure they have liquid water *at all times* by using a heated waterer of whatever type.
(wrapping heat tape

I would be awfully careful about that (read: you will never catch me doin' it). Even assuming this is a metal waterer we're talking about, heat tape is not really designed to be used on a) thin metal that b) does not have water on the other side of it (as your waterer will not once the level drops).
It seems strange to be saying this in July but folks, heat tapes are a MAJOR cause of barn fires, and if you absolutely have to use one, use one made in the past few yrs and follow the manufacturer's directions exactly.
Actually aquarium heaters are not all that cheap these days
But the main issue is that their thermostats are set wrong for winter-water-heater purposes. THe lowest you can generally set them is for about 60 F. Whereas what you want is to be able to set the thermostat for about 32-35 F.
If you go with the thermostat set at tropical fish temperatures, you will be wasting large amounts of electricity and also you will be humidifying your coop air an awful lot. And because the great majority of the aquarium heaters these days have integral thermostats you could not really remove it and replace with a more appropriate thermostat.
Also, an aquarium heater will be TOAST, and also possibly a fire hazard, if it runs dry even for just a few minutes. So you would have to have some way of ensuring that it remained submerged in a considerable volume of water 100% of the time.
A heated dog bowl only costs like $15