I got the same issue, but I solved it by building a desalination system at home. After the water is cleansed by the oyster, the water gets extracted leaving the salt behind which can be added to a new batch of fresh water. Saves me a few salt trips to the store.

The elctricity bill is a tad bit higher, but I plan to add wave generators to the oyster aquarium so I can extract power from there just by lowering and raising an inflated ball through the water line. Should also help disturbing any deposits in the tank