If you own a Jupiter Science / Emco Athena ionizer then you will know about this useful filter. It acts as a pre-filter, removing contaminants from the water before the water is passed through the Biostone filter. This results in extending the life of your Biostone filter as well as superior ionisation. This filter goes in position #1 of your Athena (goes inside the left filter housing door). The filter life indicator on the Athena will still continue to monitor the life of the filter and should be replaced when the digital display reads 9999, (after approximately 2,000 litres have run through the filter).