The fish have to be adults (age 20) before you can check the species or sell them. Once a fish has reached adulthood, you can either put it into the isolation tank or click on it to check the species. To sell it, you need to move it to the Sale Tank, and then click on Sell Fish. If you go to the Help from the game's Main Menu (in the Windows version), you will see illustrated instructions on how to do those things.