I don't think any of us have conclusive data on what either Valenciennea spp. gobies or sand-sifting stars eat. I do think, however, that the gobies would have a harder time getting to the bottom of the sand-bed and consuming the microorganisms living in those areas, depending on your sand bed depth, so that may be something to consider.
As well, my understanding is that many deep sand beds are not set up or maintained properly, and therefore fail to accomplish their purpose anyways. For that reason, I wonder if the addition of a sand bed-unfriendly organism would really have any effect on filtration within the aquarium.
I haven't tested NH3, NO3, NO2 levels for a rather lengthy period of time but I don't feel that the burrowing of my V. puellaris had much of an effect on the health of my tank. If there were any reason I wouldn't buy a sand-sifting star, it would be because I would be concerned about whether it would have access to an adequate food supply. And no, these aren't the same thing as sand-dollars (which really shouldn't be available in the trade at all in my understanding).
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-Quinn
Man, n. ...His chief occupation is extermination of other animals and his own species, which, however, multiplies with such insistent rapidity as to infest the whole habitable earth, and Canada. - A. Bierce, Devil's Dictionary, 1906
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