Some fish need thier water treated for clouding and some need a type of product that helps coat their bodies with a sort of "healty" slime. I would take it back and ask the people in the fish section...if they can't come up with a GREAT excuse..than you should get a new one!!! Pet stores
always take bad care of Bettas because they think that they don't need care, or space. But they do, they would rather have a large bowl that a small cup sized one, and make sure there is plenty of air getting in, the need tons of fresh air to breath at the top of the water. Good luck!!!!!!