How odd! I'd try and identify him/her and then try to figure out how the hell they got in there....If they seem happy and you're wanting to look ater them, keep the conditions that they're used to...fill a tank from the rain-butt and start filtration etc??
Not entirely sure?!
Let us know...I'd be interested how this turns out!
They look like the wild guppy look alikes from the creek where I live. It happened to me too, somehow it got into my outside pond with no fish but full of mosquitoes. Somehow 50 of the fry were in there. They are livebearers, I know that because one of the fish in my pond gave birth and there were no eggs i tell you that.