I honestly don't think it really matters where you keep it, as long as it's not sitting right next to a heater or pump that's throwing off significant heat. You also want to keep it away from anything that might damage it (pump intakes) and make sure it's not anywhere that it can slip out of the water, or end up above the water line if there's evaporation. I keep mine in the return chamber (near the bottom); it's hidden, exposed to good flow, and because it's located where I add new water during a water change, I can keep an eye on any temperature difference between old and new water as I'm filling the tank.