The Interior Department's Fish and Wildlife Service plans to announce the terms tomorrow of a concession to operate the National Aquarium, the fish collection in the Commerce Department basement that was one of the first targets of the Reagan budget ax.

Since the fiscal year began last Oct. 1, the fate of the aquarium had been in doubt. In January the 109-year-old aquarium received a reprieve to give the newly formed National Aquarium Society a chance to make arrangements to maintain it. The society is headed by Midge Baldrige, wife of the secretary of Commerce.

Plans call for its operation jointly by the association and by Guest Services, Inc.--better known by its initials, GSI, since it is the concessionaire for several cafeterias in federal buildings and snack kiosks in local parks. Word is that GSI plans to charge admission of $1 for adults and 50 cents for children to visit the modest-sized aquarium, which up to now has been free. An Interior Department spokeswoman said she could not provide details until a news conference tomorrow.

For Craig Phillips, the aquarium director, who has been there 23 years, its saving was the good news. The bad news was that he was fired from that job since, he said he was told, his GS 13 government salary would be too high for GSI to pay. He said a former subordinate, Brian Montague, was hired as general manager.

Until he finds out his next assignment or receives a RIF notice, Phillips said, he has been assigned to write a history of the aquarium and how it was saved, even if he wasn't.