are you sure they are sand sifters? Not all gobies are. When buying a sleeper, diamond, or two spot see if they'll eat food at the LFS. Then you don't have to worry about them starving to death. Both of my diamonds ere pigs. Ate anything however they both decided to torpedo themselves through the eggcrate.
I have a spotted Goby, diamond back I think, he never stops eating the sand, but he only goes down a little bit. I think you need some Conch's and stuff like that.
I'm not an expert but the conch only goes down deep enough to cover himself, I have two. I have about 20 Nars. snails that also clean the sand. I want to get more conch's soon, I like them, they really seem to work the sand about an inch deep.