may depend upon your room temperature. Here is a pretty useful chart:
http;//freshaquarium.about.com/cs/tipsandtables/l/blheatersize.htm
I think they are a tad low, but one wants a heater which doesn't go on and off a lot - which may mean it it too big.
Also, remember that you need some kind of water flow for them to be really efective. Just an airstone will serve, if there is nothing else.
Also, please do not go for the cheapest heater! Cheapies have a tendancy to stick and boil everyone. I would spend extra for a submersible heater, which will probably perform the best of any and last longer. Also it is easier covering the tank - which keeps jumpers and heat in.
To see what is out there, go to Google and Froogle Aquarium Heaters. Some of the on-line prices will be lower. But remember that is without shipping. Also, unlike your local shop, they don't take time to answer customer's questions. If is in your interest to shop from your LFS.
All the best!
unc
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