You might try keeping track of your fish breeding. You don't have to get very detailed, but it will help you try combinations you haven't tried before. Someone suggested in another thread that we tend to keep breeding the same fish over and over without realizing it.

For another approach, try taking one of the common fish (Spotanus, for example) and breeding that fish with each of your other fish, and then breed that same starting fish with the offspring of the new fish, etc. I think you'll soon have fish you've never seen before.

As for the magic fish, you must have at least one actually in your tank to get the benefit. The Magic Fish of Fertility only produces its benefit in the tank it is swimming in. I haven't specifically tested the others.