[Oxymoron]

My poor copperband didn't eat yesterday.
I checked the water and all parameters test normal. All other fish are healthy. There has been no new additions to the tank or tank maintenance done. He has good coloration and no visible parasites.
The last water change was 3 weeks ago.
He was eating very aggresively and swimming normal 2 days ago. I have had him for 2 months.
This morning, after not eating last night, he was laying on the bottom of the tank. I have pulled him off the powerhead once this morning.
Is there any thing I can do? Hes not looking good and seems very weak.
I do have a fishless 50 gal with a pile of aptasia in it. Should I consider putting him in there to try to get him eating.
Could this be an example of cyanide poisoning?
I know the moral opinion of most about keeping these fish in captivity and am looking for advice in saving him.
Thanks
Adam