BALTIMORE -- Some people travel thousands of miles to swim on a tropical reef. But AT&T salesman Terry Lay just slips across a crowded Baltimore street for a swim in a tank at the National Aquarium.

"Holy moley!" one boy on a school trip cried out as Lay came swimming into view one recent morning.

"Look at that!" said another. "Can you see that scuba diver in there?"

"The fish just kissed him in the face!" a third boy exclaimed.

The tropical reef exhibit is the biggest tank in the aquarium and one of its most popular attractions. But the tank -- swarming with about 600 tropical fish, several rays, a giant turtle and one bonnethead shark -- is also one of the most popular attractions for Maryland scuba divers, who apply by the hundreds every year to join the select group of 50 volunteers who feed the fish.

All are experienced scuba divers, and many are self-employed people who can slip away from the office in the middle of the week.

Several doctors and dentists do it. So do firemen and Chesapeake Bay ship pilots -- people who work long hours at a stretch and get long periods of time off in return.

"It keeps your skills sharp," said Baltimore dentist Bill Kreul, an avid scuba diver who started diving at the aquarium in 1982. And it makes you feel good, he added. "There's a certain amount of prestige. There's a little bit of ego in doing this . . . . Most doctors play golf on Thursdays. I come down and swim in the tank."

Salesman Lay started diving at the aquarium three months ago. "It's refreshing to take time off in the middle of the week and go diving on a coral reef," he said. Sometimes he invites clients to the aquarium to watch him feed the fish. "They enjoy it," he said. "It's very good from a professional sales point of view."

Groups of two or three volunteer divers go to the aquarium every day to administer the feedings at 11 a.m., 1:30 p.m. and 3 p.m. Also, they scrub slime off the inside of the big glass tank, slip medicine to sick fish and remove the occasional fatality -- the "departures," as they are called.

The fish are fed by hand, or else the rule of the animal kingdom would prevail. If the food were thrown in the top of the tank, the aggressive fish would eat it and the small and the timid would starve or be eaten. Fish that like to swim near the surface would feast, while the bottom-feeding fish would go hungry.

Before feeding the fish, though, the divers have to prepare their meals. The menu is the same every day and includes smelts, shrimp, squid, clams, brine fish, green shrimp, frozen peas, romaine lettuce, broccoli and a jelled, high-vitamin, man-made concoction.

"Standard restaurant fare," noted David Gross, the aquarist in charge of the big reef tank. "The fish are from the Pratt Street fish market. Really high-quality food."

The divers hand-feed smelts to the big fish, keeping a watchful eye out for Pita the turtle, who is sometimes so hungry and enthusiastic that he snaps at a diver.

The fish circle the divers' heads, dart between their legs and arms, scurry through the clouds of air bubbles that the divers breathe out and nip at the food bags the divers carry.

"The rays buzz you, and there's nothing you can do about it," Kreul said. "Behavior modification is difficult. When we had a lot of sharks, you had to keep your eyes on them all the time. But the rays were buzzing you and getting in your face, while you are trying to keep your eyes on the sharks and the turtle."

These days there is only one shark in the tropical reef tank, a relatively mild-mannered bonnethead -- a miniature cousin of the big and dangerous hammerhead shark.

The big sharks are one floor below in a separate shark tank, and nobody hand-feeds them. Their meals of raw fish are tossed in from a distance every three days. Divers enter that tank only on rare occasions, and when they do they are armed with sticks, knives and shark repellents and have plenty of assistants ready for a rescue.

Some of the food for the fish is mixed with plaster of Paris, especially for the parrot fish, who gnaw hard barnacles off rocks in the sea. Most of the smelts are chopped up, but a few are left whole for the 152-pound turtle Pita, who is, Lay noted, "a gluttonous overfeeder."

The food fish are put in larger bags, except for some of the tiny brine fish, which are stuffed into a pair of bellows to be pumped out in the faces of the smallest fish in the tank. Then Lay and fellow diver Bill Harris, a Baltimore fireman, put on their wet suits and their oxygen tanks and climb down a narrow ladder behind a reef in the corner of the tank, where the greediest fish already have gathered.

Suddenly, the divers have left the workaday world of Baltimore behind them and are floating in a tropical ocean, with fish of a thousand colors scurrying around them and the wide eyes of children staring up at them through the thick aquarium glass. "We just magically appear," Lay said. " . . . Once we enter the tank, we are the exhibit."