As for the vitamin C I buy food grade ascorbic acid (Kept in freezer in a light proof ziplock) and simply add a small amount during water changes each week.
Ascorbic acid is very unstable & over time will still oxidize even when stored in a freezer. Also, adding a small amount during water changes isn't going to act as any great supplement to a fish. While nutrients such as calcium & magnesium can be absorbed via the gills, vitamin c is best supplemented via the diet.
Carp are pretty sensitive fish, and believe it or not, they feed based on nutritional value of the bait they're offered, so there are some carp guys that are really serious about amino acid and protein content of the boilies, they truly believe the fish will notice a healthier bait.
FYI - I spent years fishing for carp along the Detroit River & various small systems that flowed into it, and I caught plenty of them on simple corn meal & molasses dough balls. The amino acid content was pretty much non existent, and those carp sucked them back like a fat kid eating M&M's. IMO, their keen sense of smell was what triggered the greatest feed response. YMMV