I used a bucket heater temporarily when i would move my 240 gallon, in order to get the temp up so that i could get the fish back in asap. It worked great but they get HOT quick and trip breakers often, so I'm not sure how they'd work for a permanent solution. If you use one, just make sure it doesn't burn a hole through your tank/pond.
An idea my brother-in-law had was to use one of those sticky heating pads for terrariums on a canister filter. Never tried it, but might be somthin to look into?