People have used several different methods to come up with all of the magic fish. The most popular seems to be the random-breeding-and-hope-for-the-best method. That's how I got the magic plants in Plant Tycoon the first time. Now I use two spreadsheets in an Excel workbook to track fin/flower types and body/foliage types. You don't do as much duplication of effort that way.

There are also several people who have posted pictures and "formulas" on the Web. A Yahoo search usually turns up several each time I try it - just search on the name of one of the magic fish you have already found.