That would depend on the size of your system, the amount of coral,clams, etc. that use up the calcium in your system. I have a 10lb co2 tank and I would guess it will last approximately 1 year before needing refilling. the ARM reactor media is around $18.00 per 1 gallon, It takes about 1-1/4 gallons to fill both chambers. So 2 gallons should last you at least as long as the co2. This is only an estimate.
You will also want to invest is a PH controller. John |