Excerpt: Ophiarachna incrassata, the infamous Green Brittle Star, and a few related kin are the rare exceptions to the otherwise reef-safe and well-behaved Ophiuroid serpent and brittle starfish class. In some tanks O. incrassata will behave for months or even years, while in other tanks they tend to catch and kill motile creatures whenever possible They are active predators, which arch their central disk above their legs to form a trap for fishes and other prey.
Excerpt: 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.
I have one, and watching it eat disturbs the hell out of me. I feed it chucks of Rod's Food. They can sense food placed on the opposite side of the tank and move through the rocks within seconds. They are fast.
Is anyone interested in mine...maybe i should get rid of it...I have smaller fish....but if you keep them well fed will they still eat a fish like a diamond watchman?