I did for a bit, but the ones I have (the 8x8x1 blocks) didn't fit very well. I didn't want them knocking about and/or potentially crumbling apart. So I put the sponge back and just put the blocks in the back of the sump floor. I've got three of those blocks in my sump. I've got no idea on my aluminum levels, but I have a leather coral that's been in my tank for a while and it's doubled in size since I've gotten it, so it's doing well, I think.I think @DLHDesign might have replaced his sponge with a marinepure block.