Columnaris is not hard to cure, NTD is almost impossible to cure. The former is a bacteria the latter is "Plistophora hyphessobryconis belongs to a group of organisms called sporozoan parasites. The disease doesn't just infect neon tetras, other similar fish can also become infected."
Zebra danios are especially at risk from NTD. So are a number of other fish. NTD affects the skeleton of the fish. Columnaris will manitfest as external white patches and/or mouth fungus. It will also likely be in the gills.
I suggest you remove the affected fish to an H tank and then treat them with an antibiotic used for columnaris. If they gave nTD they will die there. If they have columnaris, they very well might recover. By removing them you will minimize the risk to the other fish. NTD usually spreads when other fish munch on the corpse of a fish which died from NTD.