This is looking more natural all the time! At the risk of sounding critical (which is far from my intent and not my place) I wonder if there might be a way to make this scape appear to have been formed naturally. My guess is that any rotten rock from that formation would have fallen from the upper portion of any rock wall and if those rocks aren't actually heavy I might be inclined to lift them to the top / rear of the tank and let them go. Wherever they fall would be as close to a natural random distribution as you could emulate and from that point you could arrange them from a natural starting point to a less natural one but more appropriate for your tank. Attepting the opposite by placing them in an unnatural distribution and then moving them until they replicated a more natural distribution would be more difficult (for me) as I can appreciate art but I can't make art.
I'd also think for a minute about the snail shells and might try to scatter them in what would look like a killing field for snails, scattered in a way that made it look like there might be something living in those rocks that responds with prejudice anytime a snail makes the mistake of entering the zone.
Whatever you decide, that's easily the coolest back and foreground I think I've ever seen.
Thanks for sharing as you go!