Traditional advice has been to isolate and euthanize the fish as mercifully as possible. If several of your fish are getting it, you need to be doing partial (20% plus or minus) water changes with "seasoned water" for every day or two for at least a week. Your tank has gotten pretty funky. Sorry. :(

GG's analysis is what i'd go with too.

Now furan derivatives probably relate to a medicine call Furanace, also called Nifurpirinol. It is used for treating internal bacterial infections.

The following 3 recommendations are all taken from Burgess, Bailey and Excell's book. They suggest:
1. A short bath: 1-10 mg of the medicine per liter of water. There's 3.785 liters in an American gallon. Keep in mind that American tanks are seldom the size advertised - even without considering air space, gravel and decorations.

The bath would be best done in a tank other than the regular aquarium.

2. Long term bath: 0.01-1 mg per liter for several days. (I would recommend not chilling the fish. Don't feed them much, if at all, during the bath.)

3. Orally: a diet of only medicated food for five days.
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If the bactericide is put in the tank and if it kills the "bad guy" bacteria, it will probably also kill the "good" bacteria and pretty much trash your nitrogen cycle.

If it is one or two small fish you are treating, you might take a clean, soapless wide mouth gallon pickle jar. Mix the water and a proper amount of water. Adjust your guppy to that water and dump him/her in.

Perhaps float it in a tank so as to keep the temperature stable. Don't feed it much.

Don't put it in a tank with a power filter. ;)

When medication is done, gently lift the fish(es) out and return it (them) to the home tank. Without slopping, dump all the bath water down the toilet.

Leave the jar outside to freeze any bad bacteria out of business. (That is the one handy thing about winter and keeping fish.)