How big is the fish? An old fish is apt to be less active than a young one, but totally off food is a bad sign.
I agree that checking water parameters (including temperature) is the first thing to do. Maybe do a large water change as well, since that won't hurt. However, it is less likely to be water problem as the other fish are OK.
Other causes of lethargy can be an internal infection (bacterial, parasites or tb), or possibly swimbladder or other organ dysfunction. Apart from spots & fin, is there other signs such as - clamped fins, rapid breathing, swelling , cloudy eyes, etc. Also, is the fish pooping? <For a larger fish, you can also do a gentle physical - feel the slime coat & the belly - a mushy, sunken belly or hard bloated one indicates internal problems.>
If the water checks out & there is no obvious sign pointing to the problem, my preference is to try salt. Depending on how comfortable you are with salt-dipping fish, you can try salt dipping at 0.5-0.9% for 15-20 minutes (remove sooner if fish shows any sign of distress). A more gentle approach is to gradually increase the salt over 24 hrs to 0.3% - preferable in a QT tank/tub.
Salt is effective for a wide range of parasites - including ones that hide in the gills. Often a lethargic fish will perk up with salt, giving you a clue to the cause of the lethargy & treament.
