Cory sick need suggestions

When I got home today I noticed one of my spotted corys kept surfacing for air. When I looked closer, the front of his mouth is covered in a white fuzz, he's having trouble breathing, and it looks like his barbels are gone. I'm thinking Columnaris. I quickly isolated him and I want to treat him with an antibiotic. Which is best for treatment of him and the rest of my tank which includes platys, more corys, zebra danios and a mystery snail. I did a 50% water change yesterday. My ammonia is 0, nitrite is 0, nitrate is 10. water is hard, about 7.8, plastic plants, filtered with a pengiun 150 and an AC 20. I treat water with prime. I can't think of anything else....oh yeah....it's a 20 high.

You really can't see the fuzz in the pic, but I did the best I could.

Looking at pics and doing some quick research...that's what I think. I could be wrong. Yes, it could appear all over the body, and so can a true fungus.

Anyway, I just put him/her in my new hospital tank with a filter I've been running in my 20 gallon for about 2 months. I dosed both tanks with Maracyn and Maracyn II. I'll see what happens.

Like I said....I don't know what it is and any help would be appreciated.

I lost him during the night.

From what I understand, Columnaris spreads very quickly and all my other fish seem fine. I'm going to stop the antibiotic treatment in my 20g and keep a very close eye on the fish because I'm not sure what it is and I don't want to do harm to them or possibly crash my bio filter. I am going to feed medicated food to them for a week or so, just in case.

What I don't understand is the biggest reason for fungus or bacteria is poor water conditions and I do a 50% change weekly with a deep gravel vac, I check the parameters all the time and my readings are always good.

Thanks...

What was wierd was my other fish, including 6 corys, seemed extra lively this morning. Very strange. I hope everybody stays healthy.