One used Icecap CO2 Scrubber Medium Size.

Raise pH in Your Reef Aquarium with an IceCap CO2 Scrubber:

The pH in a reef tank should be around 8.3 for optimal coral growth. If you’re struggling to maintain pH or if your aquarium suffers from chronically low pH, connecting an IceCap CO2 scrubber to the air intake of your protein skimmer is a great way to increase and keep your pH level within ideal ranges.

Note: CO2 Absorbent Media is not included. You can buy some from my site here, Alternatively you can buy from Lifesupply.ca.