The fish must grow to be adults (age 20) before you can breed them. When they reach adulthood, put one of the fish in the isolation tank, drag another fish into the isolation tank, and the first fish will become pregnant.

The tutorial offered at the start of the game is a good way to learn the mechanics of hatching, breeding and selling fish. You can also refer to the Fish Tycoon Instructions, which contains a lot of good information and pictures of the various areas of the game.