Bricks in various sizes and parts of bowls too, were found from an islet in the tank.
After being informed, Archaeology Department officials visited the scene and the removal of sludge has now been suspended.
Archaeology researcher David Gayan Indika said the artifacts could belong to Anuradhapura or Polonnaruwa periods.
He said further excavations needed to be done at the place.
Last month too, a god’s statue was found from the tank during the removal of sludge.