as one hint, most crossbreeds of two common fish with one characteristic in common will produce an offspring that will live long enough to become an adult.
For example, a greenfin spotanus and an orange spotanus. or a greenfin spotanus and a greenfin beta. where you run into trouble is breeding one of these offspring with one of the common fish. Usually that results in a species that you need higher environmental research to keep. Often you'll get a surviving fish with two offspring of common fish though.

I still think researching environment is the first thing to do. but you need money. A good high priced fish is the speckled leaffish. HERE are ways to make it with common fish. Breed a bunch of these and sell them for $75 - 110 (depending on how much advertising you have and such) (click link and scroll down a bit)