Continuum Aquatics Bact Rox M - Porous Biological Filter Media for use in Ponds, Reefs, Marine, and Freshwater Aquariums, Medium, 250 ml

Comparison of the small vs medium size.The small is about1cm, may slow the filter flow too much on a weaker filter. The medium is about 3/4", not too big for nano filters. These definitely aren't for big filters, but good for a nano filter and regular size 10-20 average hang on back.It says it's proprietary natural stone. They look very dusty and brittle, not like pumice. i don't like that it's proprietary, they should tell you want type of rock it is if it affects water parameters and for fish safety purposes.