This is the #1 filter in the Ionia Super-S12 Water Ionizer. If you live in an older home or have well-water you will want to change the #1 filter every 500 Gallons.
Note: The usual case is to simply change both filters every thousand gallons, but those with extra sediment in their water will need to change the number 1 filter more often to protect the machine from internal damage to the water cell.
