I highly recommend using the tutorial the first time that you play Fish Tycoon. It's a fairly short tutorial that explains the basics of the game and is very useful in getting you going. As a bonus (in the Windows version only), you get some adult fish in tutorial mode. The Windows version's Help function, accessible from the main menu, also contains useful information.

The isolation tank is used for examining fish and for breeding. The selling tank is labeled "Sale Tank" in the upper right hand corner of the Windows version of Fish Tycoon and "Tank $" in the Palm and Pocket PC versions. There are two ways to put a fish into the selling tank: 1) drag your fish directly to the "tab" for the selling tank 2) drag a fish to the isolation tank, switch to the selling tank by clicking (or tapping) on the selling tank tab, then move the fish from the isolation tank into the selling tank.