Are you quarantining new fish for your tank, maybe
giving them a treatment for external and internal
parasites? That is probably one of those do as I say,
not as I do things until recently. However, I indeed
did quarantine some rainbows I bought at a club
auction a few years ago and they showed evidence of TB
and lived out their lives in isolation. More recently,
quarantined some stuff (not rainbowfish) that followed
me home from a national and had an awful time with
something they brought with them.
I would guess way over 50% of all commercial purchases
are coming in exposed to something, maybe many things.
If they weren't exposed at the farm, those central
flow through systems at some wholesalers do us no
favors. The illnesses you describe are all too common
with pet shop stuff, not a common with rainbows
purchased through club channels.
In some hobbyist circles, it seems that people are
observing maladies with some resistance to
antibiotics. That makes Bruce Hansen's dictum about
leaving them in quarantine to see if they die or not
seem more and more necessary.
All the best!
Scott