> I would think that a layer of potting clay would wind up going anaerobic
> at some point.
An anaerobic layer in the sand isn't as big a deal as you would think. But
keeping burrowing organisms like clams, MTS snails, blackworms, and other
aquatic worms seems to keep a build up of any problem gasses from occurring.
One thing I do different is that I don't filter the water. I just use an air
stone for aeration and siphon up some of the detritus build up when I do
partial water changes. (not all of it, the clams need it to survive) I grow
emergent plants and their root mats support various organisms too.
Moon
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