I have used the longer skinny one to heat my brine shrimp hatchery in the winter
I wrapped it around the outside.
Problem if it is under the tank will be the rim putting extreme pressure on sections of the heating element.
for max power override you can press the touch switch until it turns from green to white. then hold it down to control the power of the light from max to dim
change back to green to re-enter program mode
Its hard to tell from the pictures but sometimes silicone mess can look a lot like a crack/scratch. Have you tried to scrape it with a flat razor to see if it will come off. Its fooled me a few times. Either way I dont think petco would have a problem exchanging it.