Originally Posted by nomel
I stri my sand to qet the debris up into the water where it can be caught. My question is what is causing the heat. There isn't anything warm under my tank.
Actually don't do that. You want to vacuum the stuff directly up off the sand. Stiring it up only makes it more likely that it will get buried in the sand. Just wave your siphon gently back and forth over the sand. That'll create enough current to pick the stuff up off the sand without moving it around.
Like Sin and I said, sand is just an insulator, its naturally going to be warmer down near the bottom.