Apparently you don't need to buy the rarer eggs to get the magic fish. You can crossbreed the common fish a few times to get the magic fish. I bet it's harder, though.

My strategy is to crossbreed any combination possible. I downloaded a wonderful program from this site that allows you to enter your crosses and the resulting progeny and their price.

http://www.bobbiemelody.com/fishtycoon.htm has some excellent information. Just beware of spoilers, in case you want to figure everything out yourself.