Are you taking into account the energy loss of having to pump more water, especially since electrical energy is more efficiently converted to heat than to motion meaning that the heater is more efficient than the pump. I think that once you take everything into account such as the cost of the pump, the cost of the tubing, the huge inefficiency of basically every idea here, the cost of timers or solenoids to regulate when the heater is on and when the heating loop is on I think you are not saving any money and are in fact spending much more than just running your heater.Cooling my ballast really isn't the main goal... I'm simply trying to reclaim some of the heat/money which lost due to having the lights... I read somewhere that 1/3 of the cost of running an aquarium is from the heater!! On a 150 Gallon, I bet that can add up.
The only idea I have seen that may save you some money is to put the ballasts under the tank. If you cut some holes in the top of the stand just big enough for the ballast to rest against the bottom glass you would cool the ballast, warm the tank, and the heater would make up the difference in the temperature of the water. By putting the ballasts against the tank directly you save all of the other costs. Unless this somehow overheats the tank it should work