If you have softwater or use RO water, then simply add 15ml per 60L.
If you have hardwater or are unsure, obtain a GH reading using any common test kit. You can also read your local water report as we describe here.
Use the SKY Dosage Calculator (below) to determine dosage. For the very first dose, Water Volume = tank's water volume. GH Reading is your tank's GH level.
Subsequently, during water change, Water Volume = Volume of new tap water added. GH Reading = tap water GH level.
The best way is to first mix APT SKY in a small container with some water and add the milky solution into the tank.
If you have good flow, you can also add APT SKY powder directly into the tank. It is OK if it covers the plants.