I'm glad your tank is doing better. That is such a relief. Regarding your rainbows, it looks like they are better since they would be gone by now if they had columnaris.
Meth blue is very gentle and it will help them heal. They are in a QT tank , right? Just need a sponge filter, heater, airstone. It will kill nitrifying bacteria so you will need to do a 75% water change daily. You don't need salt with this treatment. You could do a meth bath for a few days. Then I would do a water change to get rid of the blue and see if they have healed.
Caution: methylene blue stains EVERYTHING. If you accidentally drop some, you can use some diluted bleach, or a Mr. Clean Sponge:
Amazon.com: Mr. Clean Magic Eraser Cleaning Pads, 8-Count Box: Health & Personal Care