Keith, A rivulus with a curved back would not be a normal situation. You might see it in very old fish, or ones that have been injured. Join the AKA at http://www.aka.org/aka/modules/content/index.php?id=9. Archives are at http://fins.actwin.com/killietalk/ Modify your subscription at http://www.actwin.com/mailman/listinfo.cgi/killietalk