I have found some hitch-hikers in my tank too I was cleaning my wet/dry siphon filter and took some moss off of it and before I threw it away I noticed these little guys. I put them back into the tank one was swimming against the power head current and the Angel sucked one up and spit it out and never touched it again it sank to the bottom and crawled off!DSC00168.jpgDSC00167.jpg I have no idea what these are.