That same thing happened to one of my Platys (who is now a very active adult male)!
I was doing a water change one day and noticed him swimming in the
UGF tube!!!! 8O He was also about 1/2 inch. He lived in the tube and/or under the gravel....the thing is Platys are livebearers! So as a baby fry he somehow burrowed himself down under the gravel into the UFG.
I never knew anyone else this had happened to until now!