90g - I have dozens of them in my 20 gallon reef, maybe even a hunderd or more. Impossible to count. I never feed them intentionally, I've never seen them hurting or eating anything. I've never even seen them on any of my corals. I don't know where they came from, but if you get some in your tank and you don't have anything to eat the asterias, they will reproduce by just splitting into more of themselves.
Sculp1n - do you keep starfish in a separate tank and harvest legs from them? Or does housing harlequins mean one is obligated to purchasing live "feeder" stars on a regular basis? I've never seen starfish legs for sale anywhere...