Olivier, I put sand into my new tank as well. Just enough sand to reach the top of the bottom trim. This way I don't see the nastiest at the edge of the sand. I Mix my sand once or twice every 2 weeks right before my water change. I also run a filter sock at this time in the sump to make sure I catch as many particles as possible. I don't depend on my sand to harbor any real nitrification bacteria. I mix and turn it over and the water column is a little cloudy after. I get corals that Extend their polyps like crazy during this process and it looks a mess but 2 hrs later the water is fine and the sand is sparkling.