Fish may be beneficial to your health. Not just eating them, but watching them.

Three University of Pennsylvania researchers report that quietly watching fish swimming in a home aquarium eases stress, and may offer a means of treating high blood pressure.

The fish are the key, the researchers say. People who watch fish tanks with bubbles, pebbles and plants -- but no fish -- don't benefit nearly as much.

"There is a sharp difference," says ecologist Alan Beck, director of the university's Center for the Interaction of Animals and Society. "Blood pressure drops with fishless fish tanks, but it doesn't drop as much and it creeps back up faster. With fish in the tanks, you truly get relaxation."

Lowered blood pressure is one way to measure reduced stress. A number of ailments have been associated with high stress in daily life, including heart attacks, sleep disorders and some ulcers.

Petting a dog or a cat is a known stress-reducer. One study showed that survivors of heart attacks tended to live longer if they owned a dog.

Beck, psychiatrist Aaron H. Katcher and biologist Erika Friedmann decided to see if interaction with other animals produced similar results.

They selected fish because they are such common pets. The Pet Information Bureau, sponsored by pet products companies, estimates that more than 10 million American homes have aquariums.

The researchers divided 100 paid volunteers into two groups. One at a time, they were put in a room and asked to read aloud -- a proven way to induce stress -- for one minute.

Then they were left alone with a fish tank for 20 minutes. Half the time there were fish in the aquarium; half the time there were none. A device automatically recorded the volunteers' blood pressure. Those who had fish to watch fared far better.

The random selection of volunteers resulted in the participation of some who suffered from high blood pressure. "For them, the fish were much better than for a normal person," Beck says.

The Pennsylvania findings are part of a $12,000 project privately funded by the pet products industry.