I've never used Vitachem myself but I was curious so I did a bit of googling.
Apparently the vitamins that can be overdosed are the fat-soluble ones which are A, D, E, and K. If levels of these build up too high it can cause helth problems. The symptoms are the same as those for deficiency of these vitamins, which are things like poor growth, poor sight, and a bunch of conditions with really long names...

However in fish it's rare for this to occur if the vitamins just exist in the water-ususally it happens if the fish is consuming too many vitamins.
Vitachem ingredients: Ascorbic acid, ulva and kelp algaes, plankton extract, biotin soluble brewers yeast, cobalamine concentrate, l-lysine, HCL, d-alpha tocophe-rol, insitol, folic acid, hydrochloride monohydrate
(posting them so I don't lose them

--I'll try to figure out what can be overdosed)