From the Seachem website:


DIRECTIONS: On initial use or after a major water change, use 10 mL for every 80 Liters (20 gallons). Thereafter use 1 mL for every 40 Liters (10 gallons) daily or every other day. Use the enclosed dispenser to measure 1 mL (top mark). For larger volume
tanks, note that 1 capful = 5 mL.