Yay! I've been on that site from time to time!
And I've got a few products by the same brand c: Most of my water tests for example, and some sinking pellets. But I once read a booklet I downloaded from there, about keeping goldfish... uhm, as a reason not to keep them in a bowl, they've written that the fish get stressed from the vibrations in the water caused by their movements that "bounce off" the walls in the bowl. Nothing else. I've kept my fish in a round tank, and none of them have ever seemed stressed
So I was like "wtf?", I think they should've said something more... useful and true about that d:
But, to the case, I've heard that most liquid vitamins are completely useless to add to the tank water, and it will usually "come off" the food as soon as it gets wet. So the fish won't really absorb them in any useful form, but the vitamins will just be floating around in the water. UNLESS, you add it to homemade food, to some vegetable/meat things in jelly or something, that way they'll stay where they're intended d: