This is a tip that someone gave to me a while back and it works really well.
A method to prevent tank cracking, over heating, and generally promote more even heat (without hot spots) is to not stick your UTH directly on the tank. Instead, stick the UTH to a ceramic or porcelain tile and slide the tile 'under' one end of the tank.
The benefits are:
a) Inexpensive- an 8x10 inch tile is about $1.50 (USD)
b) Prevents a tank from cracking
c) It’s movable- can be used on future tanks
d) Prevents hot spots by allowing airflow between the pad and the glass.
e) It’s easier to clean the cage (moving it etc.)
f) It’s the poor mans thermostat/rheostat. You can lower or raise the tank to increase or decrease heat. All you need are a couple different sized blocks of wood to prop up the tank or heater.