Heat, intestinal parasites, external parasites, stress, ammonia/nitrite poisoning, having a bad day, etc. Those are very generic symptoms, hard to diagnose a specific problem unless you notice something else.
We had a member no long ago who had a tank attacked by Fish TB or Wasting Disease. You may want to read some of her posts and some of the comments.
She sadly did end up losing the fish that showed signs of the disease and there was no treatment that helped. If the fish is showing real signs of wasting or emaciation and problems like hollow stomach, loss of color, and loss of apetite; I would isolate the fish just to be on the safe side as the disease is contagious and can affect your other fish.
I am so sorry but I do not know of any treatment for this problem. We can hope it is not Fish TB, Have you tested your water parameters? I am doubtful that it is water quality because you do not mention any of the other fish reacting in the same manner. If there is a chance of either internal or external parasites there are remedies like that cover both internal and external parasite infections.
The link was only to give you an idea what to look for. If you cannot get it online the local fish store should have the Clout or another remedy for Internal and External Parasites. See if you can find one that takes care of both so you do not have to get two different ones. Keep in mind that if you do not have a quarantine tank that you will be treating the whole tank, but if you have having parasite problems that may not be a bad idea.
Oh I am so sorry :'(. It is always a sad thing to lose a fish that we care about. I wish there had been something we could have done to help the little thing but if it was TB then it would have suffered so now you can have the knowledge that it is not suffering.
I do hope that you are okay and that you will remember the fish as it was before it became ill and while it was giving you pleasure.
Please let us know if there is anything we can do.
I would do a big water change to help in case he did die of an infection and if you have some Melafix perhaps it would not be a bad idea to dose the tank for a week. We do not know that it was TB and perhaps it will help)
hI ther fish_r_friends ..my clown loaches had the skinny disease(internal parasites) about 2-3 months ago,I used sterazin made by waterlife and that worked for me... ;D ..my clowns have made a full recovery and haven't been better..they have grown 3-4 times the size since then..hope that helps u out a bit.. good luck ..