‘Red swamp guppy’
These are a very pretty and very tiny guppy relative. I worked around my soft water problem by keeping them in an intentionally slightly-salty tank (not precise, but maybe something like 5ppt, not so salty that Najas grass couldn’t grow there.)
In my tank they weren’t especially prolific, but a few fry did show up among the plants. I didn’t try to save or raise a large number of fry, and the females would have been too small to keep in a standard livebearer trap anyway, it could scoot right through the grating.