That happens more than you'd think! The isolation tank is used for breeding and examining fish. To sell fish, you need to put them in the selling tank (either by dragging them to the tab that has a "$" on it, or by putting them into the isolation tank, switching to the selling tank ($), and then taking them out of the iso tank). You can use the selling tenk for breeding and growing fish, but keep in mind that any adult fish that are in the tank when you tap the Sell Fish button will be for sale.