The cheapest (and usually easiest) way to get those fish is still from buying common eggs. Unlike getting seeds in Plant Tycoon, you can keep buying common eggs until you get what you want. If you really want to try to get them by mutation, there is a version of Borg's Breeder for Fish Tycoon in the Guides. It's laid out in the same way that the Plant Tycoon charts are, and the mutation works the same way.