Hi, Gail! Welcome to LDW Addicts Anonymous!

You can certainly use your selling tank as a working tank. Most people do that to give them more room to operate. A good strategy is to (for the most part) keep your babies and less important fish in that tank. The babies will never be sold, and you won't lose that prize Magic Fish if you accidentally hit the Sell button (trust me - it happens)! Other than the risk of accidentally selling fish that you intended to keep, there's no down side to using the selling tank for breeding operations.