I'm going to be making a bog filter for a pond. Everything I've read says to use larger rocks for the first layer and then 3/8'' pea gravel. I would think the crushed lava rock would be better because of more surface area for biological filtration. The only draw back I could see is the lava rock getting clogged with dibree. I don't think that would happen though, would it?? Also for the larger first layer of rocks is there a certain type of rock that works best (like not changing the chemistry of the water)?