Can you describe the worm a little better?
My Initial Thoughts:
1.) Planaria (but these are MUCH smaller than half an inch - maybe 1cm)*
3.) Anchor Worms*
4.) Nematodes (Maybe? This is usually due to overfeeding, though - and you have no fish in there to feed, right?)
* = None of which you want.
Do you have photos, or could you draw it? Does the worm have little arms, how does it swim? If you turn off the light, does it come out, or does it matter? If you syphon the gravel, do you get a lot of 'em, or can you suck it out?
I dunno, I'm not big on aquarium parasites, unfortunately. If I had more information I'd research it for ya :\
I'd personally recommend NOT adding a fish right now, just in case.
Last edited by Torat; 03-26-2011 at 06:20 AM.