how deep does onyx sand need to be for a suitable substrate? min, max?

The package recommends 2", but I like it deeper myself -- 3"-4". There's no
max, really. (But too deep a substrate looks funny!)

how many sqare inches will a 7kg bag cover?

It says on the bag, but I forget.

I bought seven bags for my 75 gallon tank (18" x 48" footprint). It wasn't
quite enough, and I bought another bag. Eight bags of Onyx covered the bottom
of my tank to a depth of 3"-4".



Leigh

http://www.fortunecity.com/lavender/halloween/881/