I'm going to pass on the conch and big hammer, as I'm pretty sure neither are great for a nano.
The tank is 8 months old, and I think part of the issue is removing desolved organics. I added a Lee's skimmer about a month ago, and water quality is steadily improving. In fact, after writing my original question, I've found my sand looking better and better for longer periods of time.
I think my approach will be to add 1 or 2 marg. & cerith snails (each), and wait again.
Thanks for all the answers.
Steve