This is the Pre-Filter for the Progressive Filters used on the Arctic Spa Custom Series from 2020 through present. The Pre-Filter is used if you have contaminants in water or if you water is cloudy. To install the Pre-Filter, remove the blue filter floss at the top of the Pro-Filter. Then install the Pre-Filter in place of where the filter floss was located. Put the entire filter back in to the filter housing with the skimmer on top. You will see water going over the top of the skimmer when the pump is on and it is in the filtration cycle. Once the water stops going over the top of the skimmer, you need to remove the Pre-Filter and replace it with the regular filter floss.