For the Aaimation I rigged a complete fish with all the animation controls. Actually a so complex rig and also a detailed model wouldn't really been needed. But because there was a little time for it, we did it anyway for future jobs, Andreas Fuss said, and so we then once could recall on the complex rig model again.

The animation itself for the fishes came mostly by a animated spline IK curve where the fishes itself were constrained on the bones and were set in between two joints. So if the chain bends the fish were deformed as well, following the chain in biting each other in the tails.

To achieve that, i constrained the Head on the chain link (bone) ahead ant constrained the tail on the link after, so the fishes turned like the joints.

Also for obtaining a better and natural motion for the fish chain itself, i used a hair curve with transform and rubber constraints to define the line and authentic motion of the chain. So waves along the curve could be better animated and finally looked more credible than just using a standard curve animation with weighted clusters or similar curve deformers.
Even the fins of each of the fish's body where converted into hair curve driven splineIK's so they bounced or floated with the fish's Motion, and did not had to be animated by hand. what would be in that case a vast amout of work to do it for any single fish.

Finally there were rendered some passes like Beauty, Occlusion, Z, and also a Caustics Layer which made travel some overhead caustic light from the water surface over the fish's back.

References: see commercial on the südlich-t OHG Homepage