I have a Nature 2 that was installed with the pool 3 years ago. I still use it. I NEVER had an algae problem with my IG 18x36. My water is pretty much crystal clear all the time. HOWEVER, I do stay on top of my pool chemistry too and don't let the chlorine levels drop as low as they say you can. So I can't sit here and say that the Nature 2 is highly recommended cuz I just don't know. I did buy 2 cartridges last year on Ebay for $75 each...