Mine hangs out by the thermometer - it's on the opposite side from the heater. He's there mostly because of his bubble nest (he's an old hen) and also because hiding behind the thermometer means he's "invisible," hehe.
Do you have any hidey-spots for your fish? And is your thermometer by the heater or somewhere else?
One of my fish (the one with the thermometer on his side) just loves it, swimming up and down and around it. Small spaces, I guess. If I take it out, like for water changes, he looks all over for it. It's his favorite toy.