You need to separate him with a single female of his color and capture her fry. Some of them may show that pattern in which case a female from the same batch may possibly be carrying the gene. It would be worthwhile trying to breed one or more virgin females from that drop back to him. Again, monitor the fry for the feature. When you find the female that is dropping lots like him, you can use her fry to breed back to your male. If you keep working at it for a few generations you may be able to isolate a pair that consistently produce that crown tail look. At that point you will have your first generation breeding pair for setting up a breeding program. With that characteristic, I would get together about 1000 before selling any to anyone. That way you could go around to your local fish stores and sell a bag of 100 or so to each store. A year later there will be so many breeders using the strain that they will no longer demand much on the market, but for a short time you will have a money maker.