Well, you asked
They are possible predators, meaning, do not have any if you are going to throw a tantrum if something is killed. Meaning also that the behavior of most brittlestars (and there are over 2200 species) is unknown and so they are possibly predators. Ditto for many types of crabs and shrimp, also possibly certain anemones...maybe some corals...certain seastars, fish, etc.
Yes, they (the GREEN BRITTLESTAR) are opportunistic predators and may or may not, depending on the individual, the tank, the feeding, etc take to predation. You have watched some GREEN brittlestars eating fish...but not every single specimen has or will do this. It depends on many factors, but regardless, the green brittlestar are known predators in the wild. However, not ALL brittlestars, and I was talking in generalities, are known predators. They are not definitely ALL going to eat things, but several might.
FWIW, I have 11 brittlestars, including 3 large greens, in a 45g tank...and I do not believe they have eaten anything. If they have, then it is nature, and I accept that risk when I decided to combine these animals in potentially unnatural conditions, with unnatural tank mates. But by all means, if you don't want to take this risk, then DO NOT KEEP BRITTLESTARS (also the list of animals above), and of which, IMO, are possible predators.
Will make sure they brittle stars get spot fed. How often would you say? Daily?
