I've tried a lot of heating ideas, and aquarium heaters work, but their thermostats are not very accurate. However, they are intended for submersion, and are rated and certified for that application. Black and white processes should not be too fussy about temperature. For fussier color processing (film development) I had to purchase a lab heater from Cole-Parmer for about $175. It is accurate, although a little fussy, and the least expensive reliable solution I could find. Be sure to observe all electrical-code regulations and safety procedures.