From Seachem's website: "Purigen® is a premium synthetic adsorbent...a unique macro-porous synthetic polymer that removes soluble and insoluble impurities from water at a rate and capacity that exceeds all others by over 500%. Purigen® controls ammonia, nitrites and nitrates by removing nitrogenous organic waste that would otherwise release these harmful compounds. "
From my limited experience (7 months using it), Purigen has been a benefit to my small system (24g). I started using it just after I cycled the rock, and it lasted about 6 months before needing "recharging". Because it is a synthetic polymer, it can be "cleaned" and then reused. For the first 5 months, my water parameters were spot on, crystal clear and had very little nuisance algae.
At the 6 month mark (december), I started to have water quality problems. I pulled the bag out, recharged it and a few days later put it back in. Water quality has improved and my CUC is catching up with the algae. I'm still getting film on the acrylic, but most of the diatoms and other algae on the rock is gone.
I do a 25% water change every 2 weeks. I could probably go longer, but prefer to take preventative action as opposed to reactive. I don't have space for a sump, and all the great gadgets to help keep the system clean (really wish I had a refugium) so I found this to be a great solution. I am toying with the idea of removing my mechanical filters all together and only run the Purigen and Matrix.
The other benefit, they sell it in a 100ml factory sealed filter bag, intended for small systems. Very convenient size for the back compartments of a nano cube.
https://www.amazon.com/dp/B0002A5VK2/ref=cm_sw_r_awd_gUKNwbK890BRE
