Hi. I did a lot of searching on this and didn't really find any good answers. The only reference I've been able to find was on this web page, and it doesn't really give a good answer:
http://www.masla.com/fish/hlle.html
“One version of this theory is that the carbon could be causing nutritional deficiencies by adsorbing vitamins or trace minerals either directly from the water, or by adsorbing molecules that the needed nutrients could then bind to. This theory, however, has several problems: There aren't any vitamins in tap water to begin with, and minerals like calcium, phosphorus, and iron should be available in large quantities in most prepared fish foods. Additionally, it is reported that fish rely on food for nutrients, trace and otherwise, making the presence of nutrients in the water irrelevant (4) although fish have been shown to be able to extract various minerals from the water (calcium, iron, zinc, etc.) “
It sort of states that vitamin removal by carbon might be possible, if the vitamins are in the water - but it sounds like it's only a theory. I would think your fish would get enough vitamins from the food you feed? In any case, if the fish is healthy then continue the vitamins I guess. I've never added vitamins to any of my tanks, so I don't know much about it. Have you tried to find an internet site for the vitamin supplier? They might have information there, or a way to email their customer service department with your question.