Οπως υποψιάζεσαι, πιθανότατα είναι swim bladder. Αλλά δες και αυτό:
"Two days ago I noticed one the rainbows swimming almost vertical with head down and tail up. He is also using his whole body in a wiggly motion as he swims."
http://www.wetwebmedia.com/fwsubwebi...ainbowdisf.htm