The filter medium Purigen is a synthetic resin that makes the water crystal clear. It doesn’t only clear the water, though, but also removes organic nitrogen compounds. After some time in use, Purigen will turn brown to black, which is an indicator for its absorptive capacity being exhausted, and indicates the need for regeneration. You can regenerate and reactivate Purigen with (unscented) common household chlorine bleach.

Bagged Purigen in different states of saturation. Left: new, middle: after slight use, right: after long-time use.

Regeneration

You basically proceed in a similar way as if you were cleaning glassware, as explained in this article. The manufacturer Seachem from the USA recommends a 8,25% household chlorine bleach for regenerating Purigen, and to dilute it with water by about 50%. The common chlorine bleaches on the German market often only have a concentration of 5%, therefore you can use these lower-concentrated bleaches undiluted or only slightly dilute them. When handling chlorine bleach, the manufacturer's precautions should be followed. We recommend you to wear gloves, use tweezers and to work in a well-ventilated room.

Prepare a bath for the Purigen. Pour the bleach into a non-metallic container. Use an amount of bleach that is sufficient to completely submerge the Purigen. Very handy: The 100 ml package of Purigen already comes in a chlorine-resistant bag. Bigger packing units (250 ml upwards) are sold as dry bulk and they are used in filter bags. If you use the original filter bag "The Bag" from Seachem, its chlorine resistance is a given. If you use products from other sources, you should first find out if your filter bag is resistant to chlorine. If it isn't, we recommend regenerating the Purigen without a filter bag.

Leave the Purigen totally submersed in the bath for at least 24hrs. To make sure that all the little Purigen balls are exposed to the chlorine bleach, just turn the bag over after roughly 12 hours. When you regenerate loose Purigen, carefully stir the solution to get the same effect.
The regeneration is complete when the Purigen has turned back to its initial white color. However, it is not yet operational in this state. You'll need to remove the chlorine residue first by thoroughly rinsing the filter medium under running water. No chlorine smell should be noticeable anymore. The manufacturer Seachem recommends, just to be on the absolutely safe side, another 8 hour bath in water, to which you add some water conditioner to remove the last traces of chlorine. Please make sure not to use any water conditioner that contain substances to protect the skin of the fish, since those often contain Aloe vera, which may impede the completion of the regeneration and permanently damage the Purigen. After another rinse, you can use the Purigen again.