I received my under tank heater today. It is 10" by 17". I think it may be a bit big but hopefully ok. Should i mount it under the terrarium, on the side, or on the back. I will have 2.5 inches of water and 3-4 inches of substrate. The background and sides are made with gs foam and cork bark. I also will be connecting a dimmer to the heater to regulate the tempurature. Here is a picture of the tank with the background done except for the coir on the foam. Thanks in advance for any input.