It is possible to do what you wanted, pro-rating what you are changing into the aquarium. I would suggest that you just feed your fish very sparingly or, in the case of the adults, maybe not at all.

Another thing which could be done, if you had a plant supply, is to dump a fast growing, expendable plant in there to take up some of the ammonia. I have done that with Najas, knowing that some of that particular Najas would be thrown out with the treatment water. Hornwort so often falls apart in a matter of minutes, especially with the medicinal dyes, that it would not work that way.

I would defer to the feelings and research of others here and counsel caution against using a water conditioner while medicating. On my own fish, I might still try it if signs of ammonia poisoning were evident. Better though, to cut way back on feeding.

If the problem is real fungus (Saprolegnia or Achyla), it is consuming dead tissue. Sometimes a salt bath will cause the fungus to life off. Clean water and antibiotics to prevent secondary infection have helped some fish heal.

A quick Googling of the contents of George's purchase suggests that nitrofurazone works by killing bacteria or preventing their growth. It can even be placed on burn wounds. Furazolidone, also works on controling bacteria and protozoa, to encourage healing. Potassium dichromate is a strong oxidizer. Is it supposed to burn away the fungus so the other antibiotics can protect the fish while it heals?

Like formalin and several of the medicinal dyes, postassium dichromate can be a carcinogen, Please follow the instructions closely George and do not over dose. Touch it as little as possible. On general principle, wash your hands well when you are finished. I would do that after changing water out of that tank too. :)

I often scrub down when finishing with the fish, especually if a nasty tank was involved.

Has anyone experience with those substances? Or a better understanding of what they do?

Columnaris and even fish TB (Mycobacteriosis) can be confused with fungus, certainly by amateurs like ourselves. I'm not sure we can always tell them apart. They too result from the same declining water conditions and, to the extent that they can be dealt with, are also dealt with by using antibiotics. Might as well give it a shot George.