I have a flame angel and 4 clams (3 crocea, 1 derasa). They've coexisted for almost 10 months now. One day last October or November, the flame angel started nipping at all 3 croceas all day long. For the entire day the croceas were retracted into their shells and I thought for sure I'd have to get rid of the angel. But the next day he left them alone. And the next day. And the next day. He's not bothered them since. Interestingly, that day he bothered the croceas, he didn't touch the derasa. I think the key is making sure the flame angel is well fed, that way he won't be hungry enough to try things he otherwise wouldn't eat.