Xport Bio Blocks - Freshwater Filter Media
Not sure how to really objectively test the effectiveness of these blocks. But they are very porous and I don't doubt they do a decent job at harboring beneficial bacteria. The only downside really is that they're a very soft stone and easily chip and fragment, pretty similar to chalk but breaks off in many small chunks if it scraped something even lightly. One of them also seemed to be partially buoyant so it wasn't easy to keep it buried in the substrate while it does its thing, probably due to air trapped in it.