A conch will not go and clean your tank in the manner you are insinuating.
You need to clean it yourself every time you do a water change. Agitate the sand in a small portion of the tank don't agitate the whole thing in one day, do it over a few months in small portions during the time you perfrom water changes.
Then buy nass snails, sand sifting starfish and few conch
You can buy them cheap on the internet on websites like reeftopia.com and a couple of others that I can't recall right now.