marine i think this is a serpent tbh. I always use to tell the difference between the brittle and serpents by the center disk. Serpents dont have much of any pattern on them if you look at your pic... its pretty much predominantly green. but if you look at bluezoos pic, that pattern thats on it generally are what i see on brittles to differentiate them. If theres enough food... it wont attack a fish, but one that size could easily down a full size tang if it was hungry