Here's what it says on the back of the bottle:
Use minimum 1 ball per 4 liter of water. You may use 1bottle for a 60Liter tank for best effect, but add gradually and observe for increase in GH/KH. Put the balls directly in the tank or as a filter media. It remains active for approximately 1.5-2 years. Please rinse before use.
Ok thanks I went 2-3 per 4 liters. I'm thinking of getting another bottle for my 55 gallon to add into the filter