My dog is old too. Right now 1 put 1 drop Lugol'sIodine in his water, this lasts 2 days then I switch to some RealSalt for the minerals and Vitamin C 1 gram for the rest of the week (when the bowl is empty I put the salt and C in again). Then I start all over , some people use more Iodine but I have been careful so far. BTW our dog is 30 lbs.
Years ago I did CS but I can't remember the amount.
The C I buy from Bronson vitamins.
Read about vitamin C for pets here under nutrition.