Imagine living in a glass house with no curtains or blinds to shield you from the outside world. Day after day, gigantic alien faces peer into your home, knock at your windows, and thrust humongous hands into your living room. Sound stressful?

Ironically, those goldfish and spotted sunfish you bought last month to help you lower your blood pressure are in need of some R and R themselves. That's because fish get stressed too. And stress can make fish sick, just like it does humans, says Howard Krum, head of veterinary services at the New England Aquarium in Boston. Stressed-out fish are more susceptible to disease and take longer to heal. Stress also makes breeding difficult for fish and even shortens their lives.

How do you know if your fish are stressed? "You can tell by the way they sit in the water," says Krum. "If they're gilling more rapidly than usual, that's probably a sign of stress."

To keep things calm in waterworld, consider the following.

Why can't everyone just get along?
Remember the boy or girl in school who used to torment you at recess? Well, there are bullies in the fish world too, and they can traumatize their more timid targets by poking, chasing, and taunting them into submission. To prevent turf wars in your tank, stick with species that get along. Ask the owner of your local pet store for guidance.