OK so I've had the following response from tetra today....Hello Glenn,Thank you for contacting us. Yes, it can be used with Purigen. We get this question a lot through our website Q & A. Barbara Link|Consumer RelationsSpectrum Brands – Pet, Home and Garden Division3001 Commerce Street, Blacksburg, VA 240601-800-526-0650 x. 16120 | Barbara.link@spectrumbrands.comSo along with your research and the above from tetra, that gives me the confidence to go ahead and put it in my filter. I'm going to put it in between the cirax cage and the coarse blue sponge, unless anyone has any advice to the contrary?