Once you have several fish bowls with all stars do you have to keep them or can you sell them for more..

You sell items for much less than you paid, so you are actually losing progress by selling them. Never sell, but do move items around. The next aquarium opens when you have 3 stars to finish in all of your aquariums put together. I try to move 9 fish immediately from a very old aquarium into the newest. Keeping 9 fish in all of your 3 latest aquariums allows maximum coins for feeding them, usually more than once a day. And visitors from your team can feed them for coins also!!