Lethargic Fighting Fish

I've kept a Siamese Fighting Fish in my cubicle at work for the past
year and a half with good success. But lately he seems to be more
lethargic, just sitting still at the bottom of his tank. I'll place a
mirror next to the tank a few times a week to give him some "company,"
and he puffs up for a while. But most of the time, he just hangs at
the bottom of his tank. He looks otherwise in a healthy state, (his
fins look good, no visible signs of illness) just kind of depressed.
Is this lethargy a sign of ill health? If it is just him being down,
what can I do to perk him up?

-Fleemo