I think you'd be surprised how little power it requires to run a fish tank. I tried to calculate the power consumption of my tank when I first set it up (it is a 240l planted tank that I keep at around 25 degrees, with T8 lights that are on for about 10 hours a day)
The heater is actually on very little; as Sue said, more in the winter than summer, but entirely dependent on the ambient temperature of our lounge.
Although I didn't manage to actually calculate the consumption, we had an energy monitor at the time I set up my tank and it quickly because obvious that our plasma tv and sound system consumed significantly more energy than the tank. I could see from the electricity consumption graphs when we watched a film on the tv, but couldn't tell when the lights came on for the fish tank, for example - or when I did a water change and turned everything off. Running the washing machine, dishwasher, cooking a sunday roast or boiling the kettle to make tea for guests all showed up in the graphs as well.
300W of electrical equipment isn't actually much. Most house hold appliances have their consumption measured in kW.
If you are concerned about electricity bills my recommendation (apologies for the rant but this is my pet hate) is to replace light fittings that have several halogen bulbs. The previous owners of our house have fitted them everywhere, and with 4 bulbs at 50W each, one fitting uses as much electrical energy as my fish tank. We walked around in the dark for a bit, when my husband realised how much they cost to run. I don't replace the bulbs when they die, just move them from one fitting to another so there is only one bulb in each fitting. I will be replacing the fittings when I run out of bulbs!