The photo shows two juvenile Red Sailfin Mollies (above) and a red Swordtail/Platy hybrid (below, for color contrast).
I continue to work on new molly strains. For a long time now I’ve wanted to develop a Red Sailfin Molly. I’ve tried various crosses with no discernible progress. Then, about a year ago while sorting through our Goldwag Sailfin Mollies (for more information on this strain see this blog and the Goldwag Sailfin product page), I found some fish that were reddish rather than gold. They were set aside as Redwag Sailfin Mollies (I often, with experimental strains, name them for what I want them to become rather than what they are at the time).
The Redwag Sailfin Mollies have gone through a couple of generations since then with some improvement. While processing them in May this year, I found four juvenile non-wags that had brick red coloration. Three will probably be female and one male, an ideal sex ration for a new strain. The photo shows two of these and a red swordtail/platy F1 hybrid. It demonstrates that the Red Sailfins are at least a brick red color. With luck I should be able to improve the red.
I set up the four Red Sailfin Mollies in a breeding vat. Once I get enough of them I plan to select for redder color and, as well, mate some of them to Silver Sailfin Mollies in hopes of producing a brighter red similar to the swordtail velvet red.
As I often point out, a significant part of fish breeding is serendipity. The rest is persistence.
Is the sailfin supposed to be aggressive? He chases my beta around and I don’t want to have to take him back but my beta was here longer.
Male sailfin mollies can be moderately aggressive. In your case, the male may consider your Betta a rival, another male, or he might consider it to be a potential mate, a female. Aggression toward another male and courtship of a female often are similar. In the case of aggression toward a rival, the male will dance in front of the rival, spreading his dorsal. If the rival doesn’t retreat, he will nip at the fish. In case of courtship, the male will dance in front of the female, spreading his dorsal. He will then approach the female from the side, thrusting with his anal fin (gonopodium) toward her belly.
Your male may mistake your Betta as a rival or a mate. Generally, I try to keep Bettas separate from other fish, since they either pick on or are picked on by other fish.
If your male molly is chasing the Betta and thrusting his gonopodium, then I suspect he thinks it is a female. If so, you can provide him with a couple of molly females and he’ll quit chasing your Betta.