If your tank is new(say less than 6 months), then you are experiencing what a lot of other people go through. If you could post your tank specs and water parameters, maybe even a pic, it would be a bit more easier to help you diagnose and eventually fix your issue. On the general side though:
Cerith snails are a wonder at stirring up the sand bed, but you might want to up the number, depending on tank size. I also advise against the mandarin for same said reasons.
What kind of "sand" did you add?. Was this an already established tank(barebottom) and then you added sand? A bit more info please....