NOT LONG AGO, we spent a bit of this space recalling the ups and downs of Washington's National Aquariam over the last century or so, by way of noting the Reagan administration's decision to dump the poor fish from their tanks in the basement of the Commerce Building. And in a neighborly gesture, we mentioned another aquarium in Baltimore that also carries the "national aquarium" label. But fish fans up the parkway were quick to note our erroneous assumption that Baltimore's opened last summer.

Not, so they pointed out, explaining that it would open for the Fourth of July. But now there is new word that they, too, will stand corrected -- because Baltimore's tanks won't make the deadline. You might not care, you say? But the delay may lead to a fascinating spectacle, unequaled at any time in the Commerce cellar:

Months ago, Baltimore Mayor Donald Schaefer promised that he and Frank Gunther Jr., chairman of that city's aquariam board, would "jump into the tank" and take a swim if the aquarium weren't ready by July 3. Little did they realize how events would rise to this bait, and did; last Monday, the board announced that the opening will have to be postponed until August.

Will the mayor stick to his pledge or chalk it up as just as another fish story? "I can't confirm it, said his press secretary, Christopher Hartman , "but 'i have noticed him looking through swimwear catalogs."