So I'm looking at 5 gallon tanks.. I have a spare preset aqueon heater that is 100w.. is this too much? Its the fully submersible small sized one.
What is the ambient temps of your room versus the heat you want the tank at? Either one will work and will not harm anything - if anything its overkill but it shouldn't turn on unless the temps get below 78 on the preset. Do you have a thermometer to measure the temps for accuracy?
If you are going to buy one, its best to get one that is adjustable in case you need it someday ( in the case of ich, etc )