As before I started with some rough sketches in Procreate which I then refined for using as a reference in C4D. I hard surface modelled the fish in a single piece (apart from the eyes!), and learnt a few lessons about edge flow and pulling details in a way that doesn't interfere with this (for example the gills and nostrils!).
I actually originally intended to use a simple FFD to animate with, but it distorted the fins far too much so I ended up making a rig to pose the fins in the way I wanted. I then decided to use a wind deformer to quickly 'wriggle' the whole body.
I would have loved to have time to add some SSS to the fins, and refined the animation more, but I set a 3 day limit and this was what I managed in that time. It also meant the total render time was only half an hour for 125 frames.
I comped in AE and added some little water motes with Particular's fluid system.