Guys, there are 1000s of species out there. The names "serpent stars" and "brittle stars" aren't good species names. They're descriptive terms which only tell you if the arms are smooth or spiny. The green serpent star - Ophioarachna incrassata - is the dangerous fish eater so yes, if you've got that one keep it away from fish. Otherwise your newly purchased stars are likely to be harmless to most of your tank's inhabitants.