were they used before on another tank?
The adhesive doesn't last forever, and if they've been peeled off and replaced before, I could see that happening.
Here is a possible solution (This is what some of us do) -
Place your Heat Pad inside some aluminum foil, and instead of using the adhesive on the pad itself, get a roll of that "aluminum foil tape" that HVAC contractors use. (You can find it at most hardware or home improvement stores.)
Apply the tape around the edges of the aluminum foil under the heat pad. By doing this you don't have to use the adhesive on the pad, you can easily remove and replace the pad if you need to, and that foil tape I'm speaking of sticks forever. You won't have any problems with it coming loose.