I never put anything in my nano except the ready made water for the fish store and that tank never had a problem growing softies had a ton of purple corline And my zoos and xenias grew like crazy on the wall
A. You probably won't need to dose anything in a tank that size if you do regular water changes.
B. Most people will tell you that supplements like Purple Tech are a waste of money (PT is basically just Ca). If you keep your Ca/dKh levels in check, coralline will grow naturally.
PT is no more than a re-name of Kent Liquid Reef, with them trying to get more sales from a similar product with the name Purple-Up. All are about the same, fine Aragonite, which does not dissolve in seawater as the pH is to high. Furthermore, the fine particles will interfere with your Calcium and Alk test readings, as the reagents in the kits dissolve it.