Oh, i forgot - Tetra preset heaters are crappy. I have a 50 watt which should be able to heat up a 10 gallon but it can barely keep a 1 gallon at 74
They are cheap and sold at walmart but I wouoldn't bother with them. Some people say they work but most seem to say they are crap.
For small tanks, I like the Hagen Elite 25 watt heaters- they are not as tall as most of the others and work fairly well. I use them in 2-3 gallon kritter keepers and a 5 gallon tank but you may have to order them online - i have never seen then sold at petco.
Besides tetra, the other brand to aviod is Petsmart's brand - top fin. I have a 100 watt heater of theirs and it has worked fine for 2 years but many people say they will break inside the tank?