Invader?

It's 36^C on a day off work so I'm tidying up the frog pond, which basically means take off the pants and climb in. Clear out leaves, move a couple of the water plants so they're in deeper, watch the wrigglers and think, really must get those pygmy perch - when a small fish swims past. No hurry, not worried, out for a stroll...but where did it come from?! I'm guessing it's gambusia; the weed of the fish world but even so, I know I didn't bring it and the environment around the pond is a little dry for them. Shrug, well, I hope it'll eat the mosquito wrigglers...