Copper Band butterfly Stopped eating

[asylumdown]

This guy has been my healthiest and hungriest fish for months, he was one of the first in the tank after the QT crash killed all my previous fish. He's fat, and normally eats out of my hand.

3 days ago he started ignoring food completely. He's hiding a lot and will suddenly bolt across the tank as though he's been startled. Today I put in a frozen cracked clam (his favourite) and he didn't even look at it.

I'm a little worried. There's no outward symptoms of anything, no spots, marks, nothing. Any ideas?