Terry: I was going to bring this up at some point, but since a member mentioned it here, I wanted to say that I am doing some experimentation with this issue.
I will be seriously putting it to the test shortly but preliminarily, I have maintained coraline as well as lower form inverts such as bristleworms, brittle stars and even asterinas in as low as 1.015 sg. for several wks.
All did well including the coraline.
Obviously I am extremely hesitant to bring this up here for everyone to see, so please no one try this on their home display. Soon, I'll be going full blown hypo with these same pieces of live rock and accompanying hitchhikers in my QT. This is purely an experiment and I am definately not recommending it.