i've never seen a serpent star go after live fish. i've been observing tanks where they are housed with small fish and inverts since may, and i've never seen a problem. you should give him another shot, they're really beneficial to your tank
I am at this reefing thing since 1980 I Have seen serpant stars eat small fish
they hunt at night and can prey on small fish sleeping.
his serpant star is 10" across and can easily capture small fish especially his Picasso's that are 1.5" in size that are not hosting.
you should refrain from giving advice unless you are 100% sure