I hang a nori "stick" on a fishing line so it's off the ground and just dangles against the glass. It's for the tang and rabbitfish but my 2 urchins find it at night and polish it off (diadema, and green tuxedo urchins). Also my diadema is surprisingly adept at catching spiruline (both pellets and flakes), just shoot some at him with a turkey baster and he has these tentacles that catch them. It's really cool and freaky to watch.
HTH