What meds are you using.
I would treat with anit internal bacteria med by interpet and pimafix.
Anit fungus and finrot med by interpet is no good on columnaris.
I would go in again with anti internal bacteria med by interpet and pimafix not melafix.
Lower temp by two degrees.
No salt with the clown loaches.
You could do salt baths.
Columnaris spots are hard to get rid of.
Try the med a few more days if no improvement get back to the board.
If its a bad strain uk meds are sometimes not strong enough to kill the bad bacteria, it keeps it at bay but dosn't cure it.
Keep treating don't cut the med short.
Good luck.
Ok stop the meds it not working.
Only thing left now is to do salt baths on the affected fish.
You really need antibiotic but only available through a vet in the uk.