I can't say I've actually "seen" symptoms of lack of vitamin C in my own fish. It would be a bold statement if I attributed anything to that without proof. It's because many of the symptoms are so vague, you cannot contribute it to one thing or another on a hobbyist level. Studies show things like anorexia, neurological changes, frayed "chewed" fins, increased sensitivity to shock, curved spines, those kind of things. If you would like for me to get some of my articles together for a more specific list, I would be more than happy to! I love talking about nutrition, but I realize this is a coral thread, so I'm not sure one would be interested in "fish facts".
I personally use vitamin C (or fruit) for the immunity factors. Some of the challenges they tested, were for things like lack of food, crowding, poor water quality, illness, drastic/quick changes in environment (salinity/temp/oxygen level, etc), handling in general and air exposure. All the things aquaculture fish deal with everyday, so you can see why they are so interested. They've done all this testing specific to C to make sure it is as important as it seems, before they go through the work and expense to try and figure out how to get this vitamin to the fish. They are still not happy with what's out there because it is labour intensive and very expensive.
If my fish are not eating (or not being fed properly due to training) I use Vitamin C in the tank itself. It's one of those beautiful vitamins that they can take in through the gills, directly into their blood stream. At least I can still get that into them during these times. Otherwise I give it to them in food (fruit and Stay C).
