The heater I have is a Neptune 25 watt submerisible, for half of a 10 gallon tank. So for a full 10 gallon I would go with at least a 50watt. Mine was only about $10, and I believe the 50 watt was $15. It works well. I got it at petsmart.

Normally in petstores, every freshwater fish that isn't a goldfish is tropical and needs water at least 75 degrees.