For a 5-6 gallon aquarium, you should probably get a 25 watt heater. Maybe a Marineland Stealth so it doesn't stick out too much? I only wish their knobs weren't red, if not for that it would be almost invisible on a black background.
If you are shooting for a specific temperature don't buy that heater. You can't control the thermostat and it will make your tank whatever temperature it wants. I have a 25 watt heater in my 5 gallon tank and it works fine. I used to have a few 3 gallon tanks and 25 watt heaters worked well for them, too.
texasguy, thanks for asking this. I was looking at that exact item in the Drs. F&S catalog tonight. (This was for a 1-gallon (that pentagonal thing from walmart) I was thinking of using as a QT/hospital setup for emergencies or else RCS. No gaps in the hood to speak of so this heater would have been ideal.)
The sale pitch is not very convincing--"it'll just raise whatever temp you have a few degrees" ?? Really? You can sense whether the ambient temp is 65 or 80? --but oh my, the size would just be perfect. And it's cheap.
shoulda known. File under "if it looks too good to be true...."