Zodiac Nature 2 Replacement Cartridge for Nature 2 Express

https://www.zodiac.com.au/Content/cms/products/pdf/n2%20express.pdf

this item is number 1 on that diagram if you take the link

Features
Zodiac Replacement Cartridge for the Nature 2 express that:
Automatically disperses silver and copper ions that maintain water quality.
Maintains a stronger, healthier pH balance in your pool.
Only minimum chlorine level (1ppm) required.
Reduces stinging eyes and skin irritation.

To maintain optimal performance from Nature2 Express pool water purifier, Zodiac recommend replacing the cartridge every 6 months. Simply unscrew the lid to remove and replace the cartridge.