Re: Hygrolon on background related question
Welcome to the board, Michael!
I don't believe you need to apply silicone to the foam before putting hygrolon on, but I don't use this method of building backgrounds, so someone else may have a different opinion. I think the silicone layer over the spray foam is mainly for cosmetics. You don't want the ugly (usually) yellow foam showing through in your build :-)
As for the hygolon for water transport, I think your tank is way too tall for hygrolon to be able to move water from the bottom of the tank to the top passively. You are going to need to mist it regularly to get moss to grow effectively (and maybe limit the air flow to some extent, but that will be a trial and error thing). I would not bother making sure the hygrolon touches the false bottom water. I would guess that it would only move water up a few inches but I don't have any data to back that up.
Best of luck,