Under-tank heating pads are usually made for dry reptile enclosures. With heating for a turtle tank, it’s usually best and safer to go with the submersible or external kind. These heating pads may not be able to heat up the amount of water in your tank effectively as well (water has a higher heat capacity than most reptile substrates so it will take more power to heat it up). Especially if you want to place it under the tank, outside the glass. It will even take up more energy to penetrate the glass and heat up your water. In addition, these may not be rated to be waterproof.