Two years ago, I went to New Jersey during my summer vacation(I love America:)) There, my cousin and I went stoogig around a man-made pond that belongs to my cousin's grandfather's friend. We had nets, and we used them to catch some of these tiny fish that lived close to the plant-thick surface.

On closer expection(And my memory), I seem to recall them being very similar to Heterandria formosa. Because the only other fishes in the pond were pumpkinseed sunfishes(Always there), and some trout that had been washed into the pond during a flood, this struch me as being possibly true.

These fishes were tiny and slim, very different to be called sunfish babies, which would be more rounder and fat. And they lived close to the surface. Plus, there was some size differnce, with the largest ones being fatter. Plus, the marking where very similar too. Gold base and black markings. Sorry I can't remember about the gonopodium because I only marveled at their small size.

Could they have been Heterandria formosa? But weren't formosa supposed to live in the southen US? And why formosa and not gambusia, which I read were more invasive? And could they survive the NJ winters? I've been seeing them for 3 years, during the summer when I went to NJ.

Next time I go to America I'm gonna catch a few of the little suckers(If they are formosa) and persuade my cousin to keep them

Well your fish can be anything. It can be some species of minnows or killifish that lives near the surface. Often young fry of minnow/killifish species are found near surface. But isn't that heterandria formosa aint cold tolerant? They can survive under the ice but only for few days.

impossible to say now sorry