I am considering them. If they are in a heavily planted tank how many fry would survive?
About what you'd expect for your average small schooling fish. It's possible you may have a few if it's really densely planted and you feed well, but obviously the best way to breed them would be with a breeder net or something.
Can I ask why? Gambusia affinis is the usual mosquito fish. It is colourless, extremely, aggressively cannibalistic and not very interesting. There are lots of Gambusia species, like some of the Yucatan ones, that are pleasant nicely marked little things, but affinis isn't a fish that ever appealed to me. I've watched them in Florida, and caught them many times. And I have always released them back, and never regretted it. When I did keep them (once) they were very nasty to watch. They like to nip. To breed them, I needed to get resourceful. If not, in a heavily planted tank, every single fry was eaten.
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I guess I am willing to keep the other species. I wanted to keep a fish from my area. Thank you.
There are some others. Jordanella floridae, Lucania goodei (fantastic), Lucania parva - there are fish found with Gambusia that have really positive sides to them. I caught Heterandria formosa (least killifish) with Gambusia affinis, Lucania goodei, and Elassoma okeefenokee. You have a lot of options. I had a twenty gallon with beautiful bluefin killies (L goodei - the photo above) in with leasts, and they bred really well, side by side. The fry of both species look really similar, and I found they shoaled together in tanks. That's the cool stuff to see in the hobby.
I keep Least Killifish. Thank you.