Daz i used to use the aquarium heaters and it works. Stick an old duvet over the tank.
Doesnt give you hot water but will stop the tank from freezing up.
When you go to the aquarium, have a look at the fish tanks and see which ones they use fully submerged in their tanks and buy the same ones. Some are not designed to be fully submerged.
You want to drop them in at the bottom of the tank a couple of inches up from the bottom as the heat will rise from them.
I bring reel, poles and pump and put in the bath