I'm always a little sceptical of information given on the internet - and I was a little concerned that the sand-sifting behaviour for A. rainfordi may have been based on extrapolation from other species. However, I was more convinced after seeing this: Old Glory, Amblygobius rainfordi
. This is a webpage on the Australian Museum's Ichthyology website, which would presumably have been checked by their resident goby expert Dr Doug Hoese. (He was also my PhD advisor.) I noticed in one of my books, however, that the species is associated with reefs and the silty sand around the reefs - again that suggests that particle size may be important if it's going to sift. And of course, this will also determine whether it would work as a sand sifter in an aquarium.