Vitamin C supplement?!

No - you feed your guinea pigs a balanced diet as recommended by vets, including:

"A dietary source of Vitamin C because (like humans), guinea pigs cannot synthesise Vitamin C (ascorbic acid) from other food substances. This is usually supplied sufficiently by the fresh leafy green veggies, but it is safer to supplement this with small quantities of vitamin C rich foods such as citrus or kiwi fruit. (Commerical Vitamin C supplements added to their drinking water or Vitamin C in commerical feeds are not reliable sources of vitamin C)"
http://kb.rspca.org.au/What-should-I-fee…

vitamin C is incredible inexpensive. i prefer to use pure encapsulations brand - you can find online retailers that sell it, but i dont think its available in healthfood stores. here is a link to a powder they sell you can mix in their water http://qiherbs.com/Ascorbic-Acid-powder.…

BTW: why are you giving Vit C to your guinea pigs? should they be getting this directly from food?