Not porous enough
I had the Marinepure block under my skimmer, but it crumbled apart over a few weeks. I hoped this one from Brightwell would be more durable. Well, it turns out that it's too durable. I've had it sitting in a trash can of saltwater for over a week and it still floats. This is due to internally trapped air because the porosity doesn't interconnect enough. If it doesn't interconnect, then water doesn't get through, the bacteria can't live in there, and that's the whole point of this product. This means it's similar to putting a brick in your sump/tank.
You can pour water straight through Marinepure. Pour water over this one and it just spreads across it and goes off the sides. Not a drop comes through. I do not believe this item will do what it says. It certainly can't compete with Marinepure.