I would adjust your feeding and see if that resolves the issue, or try to seed the rock with brittle stars to see if that will swing the balance away. The only time I had lots of spaghetti worms was in a tank with no brittle stars. Currently I have tons of brittle stars, and no spaghetti worms to speak of. I don't know how the two would do together, but they do seem to have similar feeding habits (hide in the rock, hope stuff comes by).I can't speak to the H. zoster, but I will say that even if copperband butterflies do eat these worms, they are tricky. They are a beautiful fish, and when they are doing barrel rolls or that zero gravity thing they do zeroing in to dart at food there's nothing like them. That said, the following is what I went through to get mine:1) Try to buy one from a hobbyist who has had it at least a year (waited a year, no-go). This isn't a fish people decide to sell unless they have one and are getting out.2) Look for a decent specimen in a LFS. I'd say one out of every four I saw were good (visually) specimens.3) If they look good, ask to see them eat. Make sure they actually swallow it and go for more. This eliminates at least another 3/4. Talking with LFS employees and importers over the years I get answers between 1/2 and 1/5 that take to eating and aren't confirmed dead in the first month.4) Once I found one that would eat I took it home, acclimated it, and put it in a clear chamber in the main tank with rocks to hide in. It ate with a marginal appetite for a around a week, then woke up dead with no sign of a cause.5) After repeating steps 1-3 again I found one that was a voracious eater (like I'd never seen before). I took him home, and put him in a 120 sump/frag tank with 2 chromis. In a week I saw him barely nibble at PE mysis (and a variety of other foods) once, maybe twice. He did clear the tank of hundreds of feather dusters in just a few days, which I'm convinced help fatten back up. After a month or two I gave up and released him into the display tank. I've never seen him eat prepared food, he just hunts in the rocks. I briefly had some aiptasia with him, and that wasn't touched, although there were still feather dusters at the time. He has slowly gained weight so I know he's eating, but I'm fairly certain the huge amount of rock is the only reason he hasn't starved.I'm told they really love live black worms, but I don't know of a local source and don't want to pay shipping for a small amount.
