Hi Wydo... There are few problems with Methylene Blue(MB) bought from aquatics shops. Fish are very sensitive to chemical toxins, and if the MB purchased at the aquatics shop started killing fish, no one would buy it anymore. Sodium Benzoate, as Ted has mentioned, is a deep acting viral killer and is quite safe to take in small amounts.
I buy Vitamin C as Ascorbic acid powder, which is the purest form. When you buy the Ascorbic Acid tablets, they add adhesive fillers which I don't like, because these fillers inhibit the Ascorbic Acid going into solution thereby inhibiting absorbtion into the body. The ascorbic acid powder is the purest and cheapest form you can buy.
To create Sodium Ascorbate I just add half a teaspoon of Ascorbic Acid to a glass of water and then add Baking Soda(Sodium Bicarbonate) -- Arm & Hammer BS is fine -- to this mix until the fizzing stops. Now you have Sodium Ascorbate in solution -- in its most absorbable form. Then I just add the drops of 0. 1% Methylene Blue(usually 3 to 8 drops depending on your needs) to the Ascorbate or Vit C solution and just drink it down.