Apollo has given you some good advice.

You will need between 25 and 35 square feet of surface area in your bio-filter for every pound of fish you plan on having in your system at full grow-out.

Kaldnes has about 259 square feet of surface area per cubic foot of media. There are 7.48 gallons per cubic foot, so that works out to be about 35 square feet of surface area per gallon of media. So, you need one gallon of Kaldnes for every pound of fish.

Oliver