I have two 2.5 gallon tanks that are currently unheated. In the day, the water gets around 80 degrees, but at night it drops into the low 60's. Obviously, this is unacceptable and I've been looking for a heater that will work for the tanks. Problem is, I'm paranoid to buy a heater without knowing that it will work for 2.5 gallon tanks.
Some recommendations for a good, reliable, relatively cheap heater would be fabulous. (:
get something that's adjustable. they make super cheap, small "preset" ones that don't work very good, stay away from those. The consensus is 5 watts per gallon so you want a 15w (it will probably say 10 gallon on it).
I use a Marineland Stealth Pro 10-watt in my 2.5-gallon and it's been working great so far; my room's fairly cold for the most part and it has kept the water steady at 80ºF -- lowest I've seen it at was 79ºF. It's not adjustable, but that hasn't caused me any issues so far.
Otherwise, a lot of people recommend Elite heaters (I believe it's their 25-watts that are adjustable and that a number of people use in their smaller, 2.5- and 3-gallon tanks) as well as Eheim Jager heaters (also offer 25-watts, I believe). You could probably find both of these on Amazon inexpensively if nowhere else, and possibly the Stealth Pro heater as well.