Cos, just for the record, I dont agree with draining/flushing the plenum.
Mike,
On your last post regarding the building. Why two layers of screen? Would that not be more cloggable, {
, dont ask me what that word is,
}. Also I would use a minimum of 4 in. of gravel.
Rogue,
I dont believe the same diversity of critters in a Jaubert type bed, is the same nor required, as in a dsb. {my view of course}.
My thoughts on the larger gravel is for better water diffusion through-out the bed. I have never measured anything, but from my experiences with crushed coral, I believe it does become ozygen deficient, despite the larger size grains. Of course saying 5mm, is only as a very top end size. I believe the Geo Marine is more along 3mm to 4mm max. I do have some of the old crushed coral that is around 5mm though.