any fish experts - fish very lethargic - is he going to die?

[wannaBeWhateverIWannaBe]

we have a small cold water tank with three fish in it. two goldfish and one other. We have been unable to identify him from any of our fish books/the net, he's kind of a mottled fish with ve long tail/fins.

Anyway it's this fish whose behavior is causing us some concern. Basically he's been lying at the bottom of the tank, not moving much, and when he does swim it seems like an effort to do so iyswim. He doesn't have any growths/fungus/anything different about his colour/body/eyes/anything and he is eating normally.

I inherited this fish from ds' nursery and I know they'd had him for about 18 months when we got him, but that was about 8 months ago.

does anyone have any ideas? is it possible that he's coming to the end of his lifespan and is about to die? given I kon't know what kind of fish he is I don't know what his normal lifespan is. or could it be something else?

the other fish in the tank are absolutely fine.

Sounds like he is on the wa yout, tbh.
Is he lying on one side or anything? If not then it could just be his time is up, but I would type goldfish health into google and go through one of the myriad of charts that pop up, as some things can be easily treated for under a fiver.

Check the water is OK, maybe do a partial change- just because others aren't symptomatic yet doesn't mean they won't be. maybe pop him into soeom kind of isolation tank, check if he is able to eat alone (in case oters swiping it- mine did that).