Used to gutload crickets with the sachet they provide for reptiles before feeding them to my jungle perch. He was always in great condition and never got sick, but then, I don't know if it was the gut loading, or my water conditions.
When I worked at Melbourne Aquarium, we used to occassionally give the fish a hit of vitamin C by stuffing a 'people tablet' into their food. Mind you, this was mostly for the large elasmobranchs like smooth stingrays, sharks and elephant sharks (A chimaera that ventures into shallow water in Southern Australia).
Personally, I wouldn't reccomend feeding your fish fat soluble vitamins at a mammalian or large reptile doseage, simply because it would poison them. Vitamins are only good in trace amounts, in high doses, they're toxic and cause all sorts of complications.
