I have what is referred to as rosy red minnows. They are very small, very hardy fish. They are feeder fish that I bought at Pet Smart for about 16 cents each almost two years ago. My pond is very small and shallow (about a foot deep at its deepest, kind of circular about 6 feet in diameter, no living plants—I’ve tried). Probably less than 100 gallons? As adults they reach 2-3 inches in length. They hide most of the time, but occasionally come out. I think they are fairly short lived, 3 years or so? I seldom feed them because they seem to ignore every thing I give them (flake, pellets, frozen brine shrimp).
I would throw 7 or 8 in there and forget about them. My little pond was empty of fish so I didn’t worry about isolating them for a week or two. I do live in zone 6b, I think. I don’t have anything else in there with them. Last summer the adults I had did give 3 or 4 spawns (kept getting little babies a lot of the summer)