How to Stop a Fish from Flipping
Select the IK handle and open its attribute editor.
1.In the Attribute Editor click on the IK Solver Attribute.
2.Scroll down the window until you see Advanced Twist Controls. Click
on it to
open the section.
3.In the Advanced Twist Control section check the Enable Twist
Controls. Choose
World Up Type Object Up, Up Axis Positive Z and in the World Up Object
field
type locator up
4. If the Positive Z does not work try a Negative Z. It dependes on
how you
created the bones and the Spline IK.
Hope this helps...