Diana said:
>so that you don't set up massive decomposition and anaerobic conditions in the substrate
Well... it's not all bad. For about 15 years now Jim Robinson and I have been putting manure, washers, steel wool, what have you under 5" of fine beach sand. They rust, but under anaerobic conditions the iron reduces to a form usable by plants. We've actually dug up pants whose roots all went for and wrapped around a washer, like a hungry beast going "Mmmmmm... iron".
As long as it's *locked in* by enough fine beach sand (1/5" of #2 silica wont cut it) then an extremely rich substrate does seem to work, and last about 10 years before healthy bunch of plants use most if not all of it up.
Besides, I don't know if you've noticed or not but if you pull up rooted water plants in the wild there is often a strong h2s smell indicating anaerobic conditions. It seems to be pretty natural.
Here's a pic of one of my tanks with this:
