I'm willing to accept it's a water condition problem, but I really do think it was something that came in with the fish I mentioned above. I've seen some thin stringy white poops in fish before, but never anything like this. I don't really have any experience with worms or other parasitic conditions. Based on what I've read, could it be flagellates I should treat with metro? Any help appreciated, or please let me know if you need more info.