Not sure what I'm dealing with (Columnaris? Fungus?)

I'm at a loss for how to deal with this stuff. I have 9 boesemanI rainbow fry/juveniles growing out in a 29 gallon tank. A few weeks ago my husband asked if we could add some cories, so we picked out six, and they made it through two weeks of quarantine with no problems. Two days after they were added to the 29 gallon one of the cories died - it seemingly happened overnight to what had been a healthy fish. Then another. This is what the disease looks like on the cories:

Based on how fast it was moving I thought it had to be columnaris.

I started dosing Furan 2 and Kanaplex, a treatment I had read about for columnaris, and honestly I can't tell if it's helping. About 4 of the 9 rainbows are showing symptoms now but instead of the back to front progression that the cories show, they get patches of white on their sides, or on the very top of their head. I've been trying to get a decent picture of it on them but they won't hold still. Salt dips (for the rainbows) seem to help temporarily, but overall the general condition of all the fish seems to be getting worse.

General specs:
Tank has been running for 6 months, originally cycled with seeded filter from previous tank. Readings are always stable. 0 ammonia, 0 nitrite, and between 5 and 40 nitrate (I usually do EI dosing but have stopped this last week).
Temperature at 77, ph around 7.4. Today's tests show 0 for ammonia and nitrite, and 5 for nitrate.

I just don't know how to beat this thing, and I'm still not 100% sure what it even is. Any help is appreciated.

 

Finally got a halfway decent picture of what it looks like on the rainbows:

 

Drop the temp to less than 75° and keep using the meds as directed.

 

Will do.

 

Its columnaris. Fungus does not just grow on healthy scales/skin for no reason. There is always an underlying bacterial or parasite issue. Fugus only lives on dead tissue or fins.

 

That's what I figured, but a little part of me had hoped maybe it would be something a little easier to deal with. Thank you for the confirmation though.