Yep it happens! I have mine for well over 2 years now and it will stay put for a year and then move! HA!
Thanks rev. I don't like the idea of them moving like this though. I sincerely thought it was gone. I searched the entire tank and couldn't see it at one point, while th carpet was looking "full". First thought was, oh crap could th carpet eat the ritteri?! Then there he was on the bottom almost underneath a rock. I thought they liked light and flow?

