By clevername3
 
This heater is great for small tanks. It is small enough to fit in even those tiny Azoo Palm Filters (which I also recommend). Easliy hidden by plants or it can actually be buried in the gravel. Suction cups keep it securely in place.
 
It is just the right size for tanks 5-gallons and under. Only drawback is that you can't set a specific temp, it just keeps the tank a few degrees wamer than the room temp It keeps my 4 gallon tank about 4 degrees warmer than the room temp, great for the winter months. At only 7 watts it will never overheat your water and cook your fish, like a 25 watt can if it gets stuck on. Also it is not glass so theres no chance of breakage!
 
It's dimensions are: 2-¼" x 5" and less than ¼" thick.