You may not need this at all. A well made grain wort with attention to pH and Calcium and good oxygenation prior to yeast pitch should have everything you need except Zinc.

Zinc is absolutely essential for yeast growth and metabolism. Zinc is required as a cofactor for the activity of all 6 enzymes classes, ie, oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases as identified by the International Union of Biochemistry and Molecular Biology (IUBMB).1 It is also involved in the structure and function of proteins and nucleic acids.

To put it simply grapes have sufficient Zinc for fermentatation but beer wort has bugger all so whilst a wine nutrient such as Fermaid AT, will certainly help your fermentation it is not ideal.

BUT...if you are re-using yeast, have a higher gravity brew or are not using pure oxygen then this is cheap insurance.

5g will be plenty for 25 litres