Looks Guppy-like

I noticed this a couple of weeks ago. i have some black mollies, some cardinal tetras, a harlequin rasbora, some plecs and a couple of yoyo loaches. i am used to finding baby mollies but this looks like a young guppy - I don't have any guppies!
It's the one that is gold at the front end, and black with a blue tinge at the rear end.

Thanks I'm not going mad then. I bought a second hand tank including a hollow 'log' a fake plant, and some fish. I then put the log and a few fish in the tank shown and kept the fake plant and angel fish in the second tank.
But this was in June - 5 months ago. I'm surprised I hadn't noticed it before. Never mind. Lets hope it lasts longer than the guppies I have previously bought from pets at home - they never last longer than 3 months! ( the last ones having died about a year ago)