Not all aquariums are created equal when it comes to sustainability. Energy used to run filters and lights is a major concern. Where the fish originate is another, as well as where they go at the end of the relationship.

The ideal eco-aquarium, balanced and self-contained, is a cherished concept of hobbyists. But research shows that energy usage for a typical home aquarium can vary widely, depending on the setup.

According to a 1997 report from the Lawrence Berkeley National Laboratory, a small freshwater aquarium—say, 10 gallons in size—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.

With increasing size, electricity costs naturally rise. A big, 55-gallon freshwater tank might use between 280 and 400 kilowatt-hours annually. Adding plants further ups an aquarium’s energy appetite, as it requires heavier-duty lighting to keep the plants alive. Generally speaking, saltwater tanks use more energy than freshwater ones, due to the increased need for pumps and power heads to create water currents. Marine aquariums can pull 230 kilowatt-hours a year for a small tank, and up to about 800 for a large tank.

Since the Berkeley Laboratory report came out a dozen years ago, there have been a few advances in the efficiencies of aquarium equipment. Using LED lights can shave off a few kilowatt-hours and newer, energy-saving pumps and ballasts have come onto the market. One equipment salesperson estimated that the amount of electricity used by aquariums today could be about 25 percent lower than in 1997 with up-to-date equipment.

Aquariums can also have hidden environmental costs upstream. In some parts of Southeast Asia, where the vast majority of the world’s saltwater “ornamental” organisms come from, fish are caught using squirt bottles filled with cyanide, which stuns the animals and makes them easier to extract from coral reefs. But the chemical can also damage the coral, as well as other organisms living in the reefs—not to mention weakening the fish so that fewer of them survive transport. Keeping fish healthy is more than an animal rights issue, it’s also an ecological concern, because the fewer animals that survive the process, the more intensive the harvesting becomes.

So when buying wild-caught fish, look for those that have been captured with hand nets, rather than chemicals. Always avoid threatened and endangered species among both freshwater and saltwater fish.

The silvery, black-striped, Banggai cardinalfish, only found in a few pockets off the coast of Indonesia, for example, has been labeled an endangered species by the International Union for the Conservation of Nature, largely due to overzealous harvesting for the aquarium trade. The United Nations’ environmental office noted in 2003 that less than 10 percent of marine ornamental species were capable of being farm-cultured.

Fortunately, sustainable collection is less of an issue with freshwater aquarium species, because 90 percent are farm-raised. Captive breeding helps reduce pressure on wild animal populations; although many conservationists argue that maintaining a sustainable trade in wild-caught organisms can be an environmentally friendly strategy if it provides economic incentives for fishermen to keep their local ecosystems healthy.

Before heading to the pet store, do a bit of homework to find out where the desired fish comes from. There are four Marine Aquarium Council-certified retailers in the United States—in Florida, Illinois, Michigan and New Jersey—where saltwater fish have been verified to be sustainably collected or cultured and then properly handled throughout the supply chain. A new licensing program should increase the number of stores supplying MAC-approved fish.

Reef Protection International’s Reef Fish Guide further assesses popular marine species based on four criteria: survivability in home aquaria; abundance in the wild; availability and potential for captive breeding; and the collection methods used. Local hobbyist groups can be other good sources of information and, occasionally, homebred fish stock.

Finally, make sure any kids in the house don’t harbor a Finding Nemo fantasy. Releasing non-native species into the wild can cause ecological problems, particularly if those species become established populations. Do not dump an unwanted fish in a pond or flush it down a toilet. Instead, find a new home for a fish that has worn out its welcome, perhaps with a local pet store.

The best of all worlds is to avoid getting into such a situation in the first place: Make sure to buy only fish species that won’t grow too big for the designated aquarium and won’t start turf wars with their tank-mates. As with any other purchase, the greenest choice will be the one we don’t have to replace.

Nina Shen Rastogi writes a weekly environmental column at Slate.com.