He's a bit of a killer, it seems. I've put in a mediumish ramshorn and two small bladder snails with him. Net result, I have one ninja bladder snail that's managed to survive. It buries itself a bit in the sand, just enough that Cobalt can't get to him, and then moves when Cobalt loses interest, above the water line. Never realized how fast bladder snails were before this. I'm kinda hoping that he's just a bully, and picking on stuff that's tiny. I'll eventually try moving some glow light tetras, a large MTS or ramshorn (gotta find one in my 55 first) and possibly some RCS.