i am using an Ocean Runner 2000 for my 30 gallon tank's skimmer and it does put out significant amount of heat... during lights on, a 6" fan turns on with the MH light... with the lights and fan on, the temperature is at around 78F... once the light turns off, the fan turns off too... the Ocean Runner pump heats the water up to 81-82F... so yes, my water is warmer with the lights off... the only other pump in the water is a Nano Stream 6025...
along with the light, the Ocean Runner pump does such a good job at heating the tank, i did away with the heater...