[Bob]

What's the greenest way to keep a home aquarium? I love tropical fish, but I feel bad about running the filters and lights for so many hours.

I've always had a soft spot for aquariums. As a kid, I sat through a lot of long, boring dinner parties at Chinese restaurants, where the massive fish tanks were reliable sources of entertainment. But not all aquariums are created equal when it comes to sustainability. There are the energy concerns you cite, but where your fish come from is also a major issue, as is what you do with your pets at the end of your relationship.
Energy use for aquariums can vary widely, depending on your setup. According to a 1997 report from the Lawrence Berkeley National Laboratory, a small freshwater aquarium of, say, 10 gallons might use as little as 90 to 120 kilowatt-hours a year to run its lights, filters and aerators. That's about as much as a typical coffeemaker uses in a year, hardly a major energy suck in the grand scheme of things.
As you go up in size, your electricity costs will naturally rise. A 55-gallon freshwater tank might use 280 to 400 kilowatt-hours annually. Adding a lot of plants increases your aquarium's appetite, because you'll need heavier-duty lighting to keep those plants alive. And generally speaking, saltwater tanks will use more energy than freshwater ones because of an increased need for pumps and powerheads to create water currents; marine aquariums can pull from 230 kilowatt-hours a year for a small tank to nearly 800 for a large tank.
A 180-gallon reef tank requires upward of 6,000 kilowatt-hours a year.
Since the Berkeley Lab report came out, there have been a few advances in aquarium equipment efficiency. You can shave off a few kilowatt-hours by using LED lights, for example, and there are newer, energy-saving pumps and ballasts on the market. One equipment salesperson I spoke with estimated that, overall, the amount of electricity aquariums use today might be about 25 percent lower than in 1997.
Finally, if you have kids in the house, make sure they don't harbor any "Finding Nemo" fantasies. Releasing nonnative species into the wild can cause all kinds of ecological problems, particularly if those species become established populations. If you find yourself needing to get rid of a pet fish, try to find it a new home or see whether a pet store will take it.