"Tannisol is a stabilizer of Italian origin. It consists of 95% potassium metabisulphite, 3% Ascorbic acid, 2% tannin. For most people, sulphites are safe in small amounts. If you are sensitive to sulphites don't use them. Dosage: max 1.3 tablets per 100 litres."
In leiu of Tannisol, you can use potassium Metabisulfite. It is in a powdered form and very easy to use.