Your tank layout is almost exactly what I plan to do with my small bowfront.
I've haven't found many palatable options for reliable small heaters:
At the moment, the tank (Mr Aqua 7.5 gallon bowfront) is heated with a 7.5W Hydor Mini heater... and it's keeping the tank at 66-68 F in a 60 F ambient room, where the actual water volume is roughly 4 gallons after adding rocks, an aquaball, and substrate. (The tank only has White Cloud Fry and a couple of Amazon Swords) The 15W Mini may be a good choice if you're willing to dig up your substrate and your ambient temperature isn't quite as chilly.
When the fry finish their grow-out and get moved to another tank, I'm going to try a seedling heating mat instead. A low wattage (The 25W & 50W models are 9.56 inches long) Eheim Jager heater does fit, but it takes up too much room for my taste. My experience with chain store brand, Marineland, and Aqueon heaters hasn't been great. I wouldn't trust their small heaters to work for more than 10 - 20 months.
Edit: The mat might not work for your setup, since the plastic base would act as an insulator as well as break contact with your tank bottom. Heat transfer through the air works even less well than plastic, especially when you're trying to warm something with a high heat latency.
Last edited by flc; 11-30-2012 at 05:57 PM..
Reason: The heating mat suggestion may not be appropriate