Calling all fish Doc's

[Seamonkey]

I have a mature 4ish year old Dogface puffer in my fish only tank that is lethargic and pretty much off food. (very unusual) His fins seem clamped, but breathing is o.k., no signs of ich, or obvious wounds. I came back from being away 2 weeks and he didn't seem right, he always greets me with smiles and rapid swimming and begging for food. He did eat a little on the weekend, but now not at all. I did a 15% water change, as I was about a week overdue on this. It did not help.
The other tank inhabitants are fine. I have a threadfin butterfly, and I know they are usually the "canary in a coalmine" but he's fine. The other tank inhabitants are:
Singapore Angel
bangaii cardinal
2 common clown
Threadfin butterfly
purple basslet

Tank is 90 gallon, been set up for over 4 years.Temp is 80, ph 8.2 , nitrate 20 (which is normal for this tank) All fish have been with me for 2-4 years.
Any idea's??!! Maybe he's old and his time's up...
Toni