People are raving about the new National Aquarium in Baltimore, what an imaginative building it is, what a great scene, what an education for the kids.

You should talk to the fish.

What we have here is a wet Hilton. The fish get attention like they never got at home, right from the start.

"A 9-foot tiger shark just flew in from Florida," said senior aquarist Jackson C. Andrews. "He didn't know what was going on. They get disoriented and they might roll over or get caught in a corner. You put them in a holding tank, lower them in a sling, and two people get in the water to set them right-side-up and swimming."

The water is kept cool, around 68 degrees, with oxygen added to lull the shark.

"Oh yes, I was in the water with him for awhile. You just want to make sure he's well-fed, and don't let your attention wander."

Are there social problems when a new shark is let into the main tank?

"Not with this guy," said Andrews.

The shark is a female, but everyone refers to it as a he. Interesting.

Last week Andrews caught several small brown sharks off Delaware with a gill net, a vertical underwater barrier that snares fish by their gills. "Oh sure, there are lots of sharks right off Rehoboth, big ones. If people had any idea . . . "

Most of the aquarium's 5,000 fish live in the huge ring tanks that people walk past on a spiral ramp rather like that in the Guggenheim museum. Every morning everyone gets a big breakfast of Purina fish chow, and three times a day divers enter the tank to personally feed certain fish a luncheon of smelt, shrimp and squid. Then someone squirts food from a squeeze bottle into the crannies where only the tiniest fish can go.

"We skate whole fish across the water for the tarpon to snap up," Andrews said. The four-foot tarpon's mouth is drawn down in a chairman-of-the-board scowl, perfect for snapping up.

Fish also get green vegetables, lettuce, spinach, broccoli ("a lot of the bright colors come from that") plus protein supplements, vitamins and paprika (also for color), all pressed into gelatin cakes "so the little guys can pick on it too."

All day the big sharks -- definitely the stars of this show -- hang around the feeding stations. They are all kept well-fed, but occasionally one will absent-mindedly inhale a little passerby.

"We have fights sometimes, not often. There's a lobster in there that's getting a little wild and bit some guys. You get to know the personalities."

Baltimore is so far from the open sea that harbor water is only a third as salty as sea water, so salt and other elements are added when the 600,000-gallon main tank is filtered through the cleansing system. The 250,000-gallon dolphin pool is hyped with regular table salt, because "if dolphins aren't buoyant enough they have to keep swimming all the time and get exhausted and sink."

It's no picnic, being a fish.

The new National Aquarium claims 1.5 million gallons, making it the world's largest aquarium in that sense, but Chicago's great Shedd Aquarium has it beat for square footage and still insists it is the world's largest inland indoor unit (and the most stylish, with its 1930-vintage marble decor). Also right up there are the New England Aquarium in Boston, the 10-year-old prototype for Baltimore, designed by the same firm, and the New York Aquarium, built on Coney Island in 1957 to replace the famous old place on the Battery. Then there's Steinhardt in San Francisco, and the various Marinelands and other commercial oceanariums. And, of course, the National Aquarium here.

Which brings up a sore point. Buried in the Commerce Department basement but run by Interior, the 108-year-old National Aquarium, with 65 tanks and 27,000 gallons and 250 species including a garfish who has lived there 17 years, faces extinction unless Congress assigns it to the Smithsonian, according to director Craig Phillips. In the Kennedy era a $10 million master aquarium was planned at Hains Point, but that was dropped in the Nixon years, and now, under Reagan, the coup de grace seems near.

What really hurts is Baltimore usurping the "national" title.

"Senator Mathias put that over on Congress in '79," said Phillips. "It was a rider on an entirely different migratory fish bill. If Baltimore can call itself the National Aquarium in Baltimore -- and it isn't even federal, it's a city project, federal workers can't even work there -- I don't see why all the 50 or so public aquariums in the country can't do the same thing. The National Aquarium in Niagara Falls, and Miami, and so on."

Besides, he added, the National Aquarium in Washington is free, and Baltimore's costs $4.50.

The financing at Baltimore, by the way, is quite imaginative. Local industries are encouraged to contribute (Black and Decker tool company gave a sawfish), and citizens can adopt a fish, anything from $3,000 for a shark to $15 for a French grunt.

Much of the genteelly huffy rivalry among aquariums has to do, not with size, which can be measured so many different ways, but with quality of service. Serious aquariums tend to sniff at oceanariums with their razzle-dazzle dolphin shows. Baltimore lists "recreation, education and research" in that order, and certainly the emphasis seems to be on entertaining the eye and ear, with photo murals (a whole wall full of fishy eyes, each one a foot across) and whale song recordings and the Maine seashore exhibit with a real tide where children on school tours can actually pick up a horseshoe crab.

But you can also be educated almost without realizing it. Live dioramas demonstrate how fish eat: how they dig, pick, suck, slurp, shock, sting, filter, grasp or root. And how they travel: by fin, by tail, by jet, by swiveling, swishing, swirling and swarming. And how they defend themselves: by stinging, hiding, clouding, or simply running in schools. The parrot fish coats itself in mucus when it goes to sleep.

Oh yes, fish do sleep. Some of them lie right down on the bottom, resting on their sides. And they get sick. At the moment, some bug is going around among the anemone fish and they have been quarantined.

There are other creatures here, including puffins, long-distance fliers who must feel cramped behind their window, and exotic birds in the rain forest on the top floor. ("So far, only one person got hit," remarked Andrews, "a bald gentleman. Life's no fun unless there's some risks.") Scientists are here too, not so much to be studied as to study water quality problems under a $52,000 foundation grant.

Most of all, it's just fun to watch. It's better than a New York street corner: gorgeous blue fish, yellow striped ones, basketweave black-and-silver ones, streaked ones, spotted ones, goggle-eyed ones, horse-faced ones, silly ones like Rowland Searles airships with bloated bodies propelled by furiously busy little matchstick fins, frightened fish, fierce fish, fish that try to hide in the rocks, rays that lie on the bottom, and above them, cruising, cruising, around and around, powerful tailfins negligently twitching, painted-on eyes staring, slanted deadly mouths hanging casually ajar, scarred sleek backs rippling . . . the sharks.

And they all swim counterclockwise. Why is that?