Yeah columnaris is another yucky bacteria, can be hard to treat. I've had run ins with that nasty one in the past. In my experience, it happens most often with new fish during the early weeks. Have had a couple bettas with it too.I will never keep rainbowfish ever again, at least not praecox. My last praecox "raffle win" fish died very, very slowly, they succumbed to myco in a neverending QT. I treated with many things to no avail & did WCs all the time! Maybe it slowed their deaths but I ended up euthanizing the rest when I had to give up the fight after many months. No other species were involved. I too, think rainbows, of some species anyway, are very susceptible to myco. IME they were more affected by columnaris too...None of my bottom fish got it but some rasboras did.
But still I have my fingers crossed for your other fish, if not the rainbows. 'bows do seem more prone to some diseases than other fish species. Both diseases are high heat sensitive IME. There's only so much we can do about that. I have & use AC here in CA but not much in MA (the slow version of columnaris infection).
I researched like heck when QTing. It seems like the salt myco version is more people transmissable but not too serious even then, most times...
Sad sigh, good luck!
Noticed one of my bleheri rainbows today with a sore on the pectoral fin, but it looks more like a cut than a lesion, I'm watching for a day or two before making the call on it. I think he may have swam into the powerhead for whatever reason. Only thing I can think of that'd leave a cut on the side like that. Or, it's a lesion and I'll have to cull the little one too. I want to give it a day to get a better look at it.