I think fish suffer of hypovitaminosis Vit C., since this vit. very sensitive to diet manufacture conditions and to insure its effect, could we add it directly to the water.
Vitamin C is known as a potent antioxidant, so it will be destroyed very fast in water by several oxidative compounds such as dissolved oxygen, chlorine and similar compounds.
However, confirmation of symptoms is highly recommended because they are widely interfere with other causative agents, you may find some useful ideas in within the attached links.
As Mr. Najim said active organic substances like vitamins are highly unstable in water especially in warm water regions. Some years ago we had to fight against fish diseases. The best results have been achieved by adding antibiotics to fish feed. Taking into account our results I strongly recommend you to add vitamin C (as well as other chemicals/medicine needed) to fish feed. The best results can be achieved if you use air-dried on farm made fish-feeds.
Vitamin C is water soluble, if we add this vitamin directly to the water, this effect less efective to promote its immunity. The best methode can be achieved by mixing this vitamin to fish feed.