Ahh, forgot D: Sorry^^; Multitasking....
For my 2.5 gallons, I use the Hydor mini....though unless your room temp remains pretty stable, I wouldn't recommend it because its not adjustable. Just look for the smallest adjustable submersible heater you can, I don't know brands that well....but so long as its fully submersible, adjustable and not huge, I think you should be okay.
Ahh, my horse is pending sale currently xD Do you do western or English? ;)
I have two giant 50 pound lap puppies too
I'd love to have more animals, though of course moms not big on that. However, I lead a rather lonely life of homeschooling and the dogs aren't allowed free reign of the house yet....so its my underwater puppies that keep me company ;)