Starfish

Most starfish sold for reef tanks are harder to keep than they look. Brittle stars and serpent stars work the rock and sand for leftovers and are safe around corals, with one exception: the green brittle star grows large and hunts fish. Linckia and the other filter feeders need a mature tank with an established food supply, and they starve slowly in a clean one.
The ones that work and the ones that do not. Brittle stars and serpent stars are the useful ones: they live in the rockwork, come out to clear leftover food, and leave corals alone. The green brittle star (Ophiarachna incrassata) is the exception and will hunt small fish at night — it is the one worth avoiding no matter how good the price is.
Linckia and Fromia are not beginner animals. The blue linckia in particular is one of the highest-mortality animals in the trade, and the reason is that it feeds on biofilm that only a mature, established rock surface produces. In a tank under a year old it slowly starves, and the visible signs — a deflated arm, a white patch, tissue breaking down — appear weeks after the damage is done.
Acclimation is the other half. Echinoderms have no way to regulate against a salinity change. Drip acclimate over an hour, not fifteen minutes, and never net one out of water into air if you can avoid it.
For a crew that does the work without the risk, snails and hermits are the better buy — see the clean-up crew guide.