Well if you run your net along the heater and the bubbles come off then it is the outside. Usually if the heater is cracked you will see some condensation on the inside of the glass. If it was cracked you would probably see some stray voltage in the water. You can check this by putting the positive lead from a multimeter in the tank (I recommend using a paperclip touching the lead so that no contaminates get in) and the negative to ground (bottom prong in a three prong outlet). Hope it helps.