First, be sure the fish did not jump, or ended up in the overflow/sump, etc..
To answer your question, a fish carcass can be completely taken care of in a couple of days so that there are no traces left. Aside from the snails and crabs, you likely have a population of bristle worms and other critters which helped consume the dead fish. In my 120, on the rare occassion a fish dies, I will normally leave it in the tank for the clean up crew. A 6 inch foxface carcass was gone in less than three days. The worms, nassarius snails and crabs had a feast.