THE NATIONAL Aquarium, which opened last week in Baltimore after some delay, turns out to have been worth the wait. Come to think of it, the month-late opening was a treat on two scores. First, the delay made possible the memorable sight of Baltimore's mayor, William Donald Schaefer, carrying his toy duck and swimming among the seals at the aquarium after he promised to do as much if it were not open by July 4. And second, the delay made the anticipation all the greater. As it happens, the aquarium is an experience that easily satisfies all expectations.

As a visitor takes an outdoor escalator up and into the aquarium, set on the edge of the city's newly developed Inner Harbor, tubes of bubbling water surround him. The sensation created by the bubbles is an apt introduction to the aquarium. Then it is up, over, around and down as the museum's clever design draws the visitor within. After the entrance is the sight of a mammoth whale skeleton, followed by tanks of live fish. For youngsters there's the "Children's Cove," in which the kids are allowed to touch living sea animals while traversing a re-created version of the coastline of Maine. Then it is on to the rain forest, where animals of all sorts prowl freely under a 64-foot-high glass roof. Finally, the visitor circles down and around tanks of Atlantic coral reef and open ocean space, which afford the fascination and terror of sharks gliding by within inches.

Washington visitors to the new aquarium can also see some old friends. Despite the gurgling in Congress over whether the fish from Washington's soon-to-be- defunct aquarium should be sent to Baltimore or raffled off to competing cities, the National Aquarium has deservedly been given the 1,049 fish. There was some talk of holding on to the fish until the National Zoo here could design a small aquarium to be part of its display. A half-million dollars has been appropriated by Congress for the zoo to begin design of that aquarium. But the reality of it is easily five years away. That is too long to storehouse our finned friends, especially when they could be on display in the wonderful new aquarium in Baltimore.