starfish don't like water with too high nitrates. that being said for most inverts. make sure no big triggers or big wrasses are present because some are known to eat them. including puffers. feeding wise, most feed on detrisus and bacteria film, abit of algae. those with "horns" i.e the knobby kinds, will consume corals. starfishes if fast enough may consume uneaten food lying around too.
brittle stars are detrivours. as said with starfishes, too high nitrates will cause them to drop legs and bristles. they do not require supplemental feeding. they eat uneaten food that lie around. if u have a brittlestar big enough, (like mine. easily 8inches across), they can extend their arm in the water current and catch floating pieces of food such as mysis and pellets.