Not really. Water by nature, unlike air, is resistant to both rapid heating and cooling, so water heated in your sump basically warms slowly enough and homogeneously enough so that when it's pumped back into your display tank it pretty much matches the water in your display tank because slightly cooler display tank water is being drawn back down into the sump. I've tested water temps both in sump where the heater is and in the display tank and there is negligible difference (on a 56 gal. tank)