On the topic of fish diseases, here's a few fun ones that might be interesting to modify to fit CD. NO PRESSURE but food for thought
Columnaris: bacterial infection that can cause white cotton lookin growths in the gills and mouth, discoloration, lesions along the body
Dropsy: kidneys fail and the fish starts accumulating fluid and puff up. The scales raise like a pinecone. The swim bladder fails and the fish can't float/sink at will
Fish fungus: an injury gets infected and grows cottony fungus along the old scars
Velvet: a rusty flaky scaly lookin disease with black spots appearing amid the rusty flaky scales
Anchor worms: worms attack fins/tail and can be seen hanging off/trailing from the fish
There's treatments to everything but if untreated can be deadly, and super fuckin contagious in a lot of cases. Consider? A ship on a hunting trip happens to bring an infected sailor on board, and the symptoms show up when they're days away from help. Maybe it's standard practice to have a first aid kit of cures on board, maybe they don't have cures on board and the crew is struggling to make it back to land before they all die :)
OOOHHH YOUR BRAIN IS LITERALLY SO MASSIVE FOR THIS ACTUALLY im putting all of these ideas in my mouth