I highly doubt the lack of an air pump would have caused this....If that were the problem you'd see him gasping at the surface maybe, rapid gill movement, that sort of thing.

Yes, that's my take too, that the air pump is an issue to fix, but not the cause.

Still, the tank has been pretty steady state for some time, and the food is all dried flakes, fed by the same person on the same schedule. So I'm at a bit of a loss how an infection was caused.

Thanks for asking about the filter, because I was going to order some new foam and charcoal and forgot until reading your post.

It's a 30 gallon tank, the filter itself is an AquaClear 50 like this one:
https://www.amazon.com/AquaClear-50-Power-Filter-Listed/dp/B000260FUM/ (listed at 200gph)

On Friday I replaced the foam and the charcoal from the filter, cleaned the algae from the glass and topped up the water. I've been doing that every six weeks or so for the past two years.

And yes, I removed the charcoal about an hour ago... I'll put in some more melafix tomorrow.

I changed 30% of the water this evening, should I do that again, and if so when?