Overweight Cory

That sounds like one of my habrosus cories. This one is also paler than the rest. But it is one of the first habrosus that I got and it is still going strong. I assume from your signature that yours is a habrosus (salt & pepper) cory too?

I use behaviour as the guide to fish health where there is a lack of physical symptoms like white spots or disintegrating fins. If a fish looks fine and behaves fine, I just keep an eye on it till something changes.
An example - another habrosus cory, a smaller one with nice dark markings, suddenly started wedging itself in the hornwort. This is not typical habrosus cory behaviour; sitting on broad leaves yes but not wedging themselves inside a fine leaved plant. I found it dead a couple of days later.