I wouldn't recommend one for less than a 55 probably, though.
Unless you have at least a 2.5" deep sand bed with a tank footprint of 8-12 square feet, don't get a sand-sifting star. They do tend to starve to death in tanks less than 100g although there are some individual exceptions as with any animal in reef aquaria. It happens all too often that the hobbyist purposefully acquires an animal despite knowing they will likely die. This unnecessarily increases market demand and collection of the animal, thus depleting naturally occurring populations and resources. Just my $.02: If there is adequate empirical knowledge available and most experienced reefers advise against purchasing an animal, it is best to leave it in the wild.
As far as eating all the microfauna in the sand bed, this is partly true. The sand-sifting star gains most subsistence from detritus, uneaten prepared foods, and animal waste. To avoid eliminating substrate microfauna, a proper setup should include a refugium or at least a structured area in the display tank that will protect such microfauna from predation. In this case, your tank's ecological equilibrium will compensate for microfauna loss. In retrospect, most reef aquarium sand beds are to shallow (<4" deep) to fully utilize the denitrifying effects of substrate microfauna.
With my sand-sifting star, I found that a standard 90g (48L x 18W x 24H) wasn't large enough. It would scale the walls and rocks quite frequently. When my 225g prop tank (96L x 36W x 15H) was finally ready I transplanted the starfish. Suddenly, the wall-climbing stopped and now it just burrows in the substrate and moves to a new area.....repeat.
To the OP, if I did a 30g my CUC would consist of an
Amblygobius rainfordi (Rainford's goby), a small harlequin serpent star, some hitchhiker polychete worms, 10 medium nassarius snails, 8 cerith, 6 astraea, and 4 trochus snails. These have all been model citizens and hard workers in my tanks. A major component of the CUC that people often omit are the microfauna (nitrobacters, copepods, amphipods, various zooplankton. Also, microfauna in your reef system make up a lot of your tank's biomass and do the majority of the housekeeping. Try culturing and feeding nanochloropsis phytoplankton (it's incredibly easy) and feed your tank in moderation. By doing this, you can more accurately simulate the natural food chain, provide free and readily available protein sources to your fish and corals, and decrease nutrient levels due to overfeeding and inevitable detritus accumulation. -----Or you could just add an adult Harlequin Tusk.
Sorry so long-winded......just took my graduation exam and still a little wound up. Hope it helps anyway.