Like smb said, if everythings good with your water... he's probably just nervous. I have 4 neons, 4 glowlights, and 3 black neons. I've recently added some taller plants and my glowlights love hiding inbetween the leafs. Your fish could just be feeling insecure because of their size. My black neons are the shyest of all my neons. They like to hide near the back of my tank too behind my rock piece, but they're doing fine and are never too shy to come out when feeding time starts