I just bought a 50w heater for my 5 gallon. It said it was for a 10 gallon tank. It was $14.99 at my lfs. Don't know what the brand is because I just looked at the box and put it back in the closet and then when I went to look at it again to get the brand name and specifics, it had disappeared. I hate closet monsters.
I think the general rule of thumb is 5w per gallon, but I wouldn't bet the farm on that.
I agree with the above statement. I have a 25w which is the minimum for a 5 gallon and on my 6.6 fluval, I have a 50w.
My 25 watt is not adjustable ( stays on at 78 degrees ) but the 50w is nice because its adjustable
50 watts is more than enough for a 6 gallon tank. I usually try to keep my heaters diagonal, if I can. A great heater to buy is an aqueon pro. little more pricey but they rock. Plus, they have no glass!
I also keep mine on a diagonal in my tank. And you want to get an adjustable because the ones that are pre-set are at 78 degrees and if, heaven forbid, you have an ich outbreak or something that requires raising the temperature, you are out of luck.