I would also suggest 100 watt is way too big. The way heaters are rated is way off in my experience. I found out the hard way that a 50 watt will kill the fish in a ten in under 8 hours! I would look for minimum size heater or if you are a DIY guy, go another way to protect the fish. Heat from the bottom using simple light bulbs can put plenty of heat in a five. Tied together with a simple temperature controller, light bulbs can be out of sight and do a good job.
Please note that this is not just based on my current warm climate, as I have also lived as far North as Northern Ohio.
Last edited by PlantedRich; 08-13-2015 at 02:10 PM.