i am very new to fish keeping so i can't tell you about using vitamins with them, but i do know a bit about vitamins. Centrum is one of the worst vitamins on the market (yes i realize this won't be a popular statement as a lot of people use them). they are so processed and compressed that your body can barely digest the coating let alone the vitamin itself. to put it into perspective, *WARNING THIS IS REALLY GROSS* in trailer parks they actually had to put specific screens on the hoses for pumping out port-a-potties because the hard vitamins would clog the system. and when the rinsed off the vitamin you could still read the "Centrum" on the side.
so although i'm not recommending that you go out and spend $50 on a multi for your fish, if it were me i would simply buy a decent one for myself (it doesn't have to break your bank account) and use a couple from it for the fish. i'm sure the ones specifically for fish are better , but if they are anything like vitamins, etc for dogs they are overpriced and the same thing you get with human vitamins (or of lesser quality).
(BTW i own and run a health food store)