Once a week and 10 gallons. I am also thinking it may be fluctuating temperatures. I've noticed at night after the lights go off, when it starts to drop the heater doesn't stay "on". Instead it does like 10 minute cycles and acts like it can't keep up with the dropping temps. Thus I have decided to get a different heater.
Penguin probably causing nitrates unless you clean weekly. Hope the heaters in the sump; on a side note make sure you buy the right size or wattage for your tank or maybe even a little bigger. Have done the cheap way and it wasn't good. Anyway have you tried to help it stick anywhere? Have you tried to move it after it was stuck? Could have hurt its foot.
You might just want to add another heater and use both of them. I have 2 on my 55 both are under watt for that size tank but I have heard to many stories where a heater will stick on and cook the tank.
Strange it hasn't landed somewhere and settled in.