One goldfish is perpetually lethargic; other one okay. What's up?

***I am not a fish expert. These are the first fish I've had in about a decade.

I have a 20 gallon tank with two small gold fish (both black moors) in it (I know a larger tank would be optimal, but I'm going to wait until they grow a bit more to upgrade).

Both fish are between 4 and 6 months old and are visually very healthy. Scales look good, neither struggle to swim or breathe, no gulping for air, etc.

However, one fish just sits at the bottom of the tank and doesn't move. Occasionally he'll swim around (doesn't have any trouble doing so; isn't slow or labored), but otherwise does NOTHING. I have not seen this fish eat in months, but he's still alive, so he must be eating at some point.

The other fish is very active, swimming around constantly. He comes up to the tank when I walk by, swims to the surface to eat, and has a really good appetite.

I've been checking my water regularly. pH is fine, temperature is fine, oxygen seems okay. I keep the water clean, and my bacterial colonies seem good (although I'm not really sure how to monitor their health accurately).

I'm just not sure what his problem is. Maybe he's just shy?