If you want to save electricity on your aquarium, you should look into what you're lighting it with, and how much you're heating it.
Heating - Remember that if you're running an AC, any energy the heater uses is approximately doubled by the extra AC runtime it causes. Actually goes for any power consumption in an air conditioned environment. Insulating your tank back and bottom, covering the top, covering any sumps/hobs, and turning up the thermostat helps a lot here.
Lighting - make sure you're using fluorescent lighting. Minimize how much you're lighting your tank and how long you're lighting your tank. Make sure you're using a timer.
Goes for the entire house. Use CFLs instead of incandescents, turn them off when not in use. No, they're not the best solution, but they're the best we have so far.
People whine that they have 5mg of mercury in them (btw, something like 20 times less than is released by burning the extra coal to power an incandescent), that they have a quarter second delay before they turn on, that they have to warm up to full brightness over about 2 minutes. Mostly non-issues. Find a CFL that you like.
I'll just throw it out there, if you use any incandescents in your house, you have no right to whine about your aquarium's power consumption.
UV sterilizers - put them on a timer.
Set any computers to standby after 5 minutes of idle - a modern computer resumes from standby almost instantly.
