Beware the Midnight Fish Eater!
One species in common use warrants a statutory warning. This is the Green Brittle Star of the genus Ophiarachna. This animal is a predatory fish eater, that does indeed do a spiffy janitorial job when small... but grows quickly, and under darkness of night can/does learn to eat aquarium fishes. This species has been documented to arch up in "sleeping caves" of captive fishes and drop down on unsuspecting meals. If you use this species, keep an eye on it, and a count on your piscine livestock.
BrittleStars Green Brittle Star
This could be what happened!