MISSISSIPPI STATE -- With low startup costs, easy maintenance and good looks, freshwater fish are staking their claim as a popular American pet.

Dr. Skip Jack, aquatic medicine specialist at Mississippi State University's College of Veterinary Medicine, said more money is spent on aquaria, fish, food and equipment than on dogs and cats.

"Fish are the No. 1 pet animal in the United States," Jack said.

Part of their popularity is due to them making ownership easy.

"Once the tank is established, feed them daily and give them periodic water changes," Jack said. "They're very low maintenance if you change their water on schedule and don't overfeed them. Most fish don't need a lot of light and get along with room light just fine."

Salt water fish have an entirely different set of requirements and are not as easy to keep.

Water quality issues are the biggest problems most fish owners face, and the biggest of these issues is too much ammonia in the tank. Jack said fish waste products and excess fish food decompose into ammonia in the water. When ammonia builds up too high, it can kill the fish.

"Everything you put in the tank has to be cleaned out of the water," Jack said.

Filters do much of this task for the aquarium. Jack said mechanical filters have a biological component as microbes grow up in the filter and break down the ammonia into a nontoxic form. He recommended under-gravel, powered filters as the best choice. These systems have pipes that run up the back of the aquarium and use air stones to draw water through the gravel. External power filters siphon water through a polyester "wool" at the back of the tank.

Vacuum the gravel about once a month to remove sediment that accumulates on the aquarium floor. Further protect water quality by doing a 10 to 15 percent water change twice a month, eliminating some of the nitrogen that builds up as the ammonia is broken down.

"By doing partial water changes, you can cut down on algae problems as nitrate in the water promotes algae growth," Jack said. "You can limit the amount of algae in an aquarium by keeping it away from sunlight and not leaving the light on all the time. Also, some fish eat algae and can be fun to watch."

Before putting fish into a new tank, Jack said tanks should be established first. To establish a tank, set up the equipment, fill the tank with water and let the system run for a week to 10 days. Put one small fish, such as a hardy goldfish, in the water initially or a small amount of ammonia to let the bacteria in the filter build up and get the biologic filter operational.

"If you set up a new tank and put too many fish right in the water, in about three days you'll have fish dying as the ammonia builds up so fast it kills the fish before the biologic filter is established and can break down the ammonia," Jack said.

Jack said a serious mistake newcomers to the hobby make is to assume that all fish are alike. In reality, they have very different preferences and requirements, including light and temperature, pH, salt or fresh water, pristine or murky water, and aggressive individual or schooling habits.

"In the wild, fish choose where to go, but in an aquarium, you choose for them," Jack said. "Most fish will adapt and live in almost anything, but that doesn't mean they'll thrive. Fish that are stressed have duller colors, reduced activity and are more prone to diseases."

The basic rule for how many fish is appropriate in a tank is one inch of fish per gallon of water. Using this rule, a 10-gallon tank can be stocked with one 10-inch fish, two 5-inch fish or 10 1-inch fish. Stocking at a higher rate requires more maintenance and may produce problems.

A final note of caution is when stocking new fish or live plants to an existing aquarium. Jack recommended owners place these in a quarantine tank for a few days to ensure that they are healthy and only buy from reputable dealers. A 72-hour return policy will not help if the new fish is sick and infects the entire tank with a disease.

Backyard ponds are another popular way of keeping fish, with koi and fancy goldfish ideal for these locations. Jack said the pond should be more than 2 feet deep if owners plan to leave the fish in it all year. At this depth, fish can swim to the bottom and survive when the pond freezes over. Break the ice to allow for oxygen exchange.

For more information, contact: Dr. Skip Jack, (662) 325-1311