This is a good question, but unfortunately, the answer is less straightforward. The reason is because there are so many variables which can affect the efficiency with which the biofilter operates. Nonetheless, I can give you the following number for Ehfisubstrat Pro. I have 12 liters of media supporting a 350 gallon, moderately stocked tank, and my water chemistry is perfect. This works out to approximately 1 liter/30 gallons. To be honest, most of us just empirically detemine what filtration system is likely to be adequate and then max out the biomedia.