The kind that stick to the underside of the aquarium.
The Farm Innovators bird bath de-icer Model B-9 doesn't need to be submerged. It is foil covered, about 12" flat round with 44watts of heat spread across it.
They're pretty flimsy but do the trick.
http://www.thatpetplace.com/economical-bird-bath-de-icer?gclid=CIvLtZrWvcoCFdU2gQodDlwIMQ
