Like 2pac said, baths or dips are usually used to accomplish this, as well as cleaning up your system. I would lower your tank temp to 75 degrees F only for treatment process and you can use Mebromin, Potassium Permanganate, Methylene Blue, or plain old salt. Don't use malchite green, that is for fungal and Potassium Permanganate can rid of both fungal and bacteria, as well as a lot of other types of parasites. Your fish may have a combination of diseases since it's system has been compromised.
I would set up a hospital tank because it will be much easier to manage, and in the mean time you can clean up your main tank and keep the water temps at 75 or lower for about a month to be safe. I wouldn't recommend just salt. I would definitely use one of the other mentioned and just put a normal dose of salt into the tank itself. What i like to do when i add salt is purposely drop it right over my fish so that it lands on him and he breathes it in through gills and mouth. He probably won't like it much, but it definitely won't kill him and it will help with his slime coat. When you set up hospital tank make sure you treat the water with some conditioners and acclimate the fish before just tossing him in. FH's are pretty sturdy, but just to be safe.
I don't know all the directional use of the chemicals mentioned, but I'm sure they have them out there. Here's some info on 2 of the medications I mentioned. I also bolded why you shouldn't use methylene blue directly into your normal setup.
Most people do salt dips or baths in buckets for introducing new fish to an aquarium and is actually very effective. That's for a relatively short period obviously and there's usually an airstone.
The biggest drawback is your fish could freak out, so pay attention to him when treating and if funny things start happening probably a good idea to remove him. I'm not a pro on these methods so maybe someone who has done it before can chime in.
Methylene Blue is effective against skin and gill flukes, ick, velvet, fungus and many external parasites and bacterial infections. This compound is very safe to use even for many invertebrates. Methylene Blue works by staining cells and disabling their metabolism.
A positive side effect of this compound is the ability to increase hemoglobin (oxygen carrying cells) by transforming methemoglobin. Because of this characteristic, methylene blue can be used to treat nitrite and cyanide poisoning.
Methylene Blue should not be used in the main aquarium as it will destroy or severely defect the biological filter bacteria. It can also permanently discolor equipment and silicate seals blue.
About 12 mg/l will treat a saltwater fish for up to one hour and a freshwater fish for up to 10 minutes. A dip can be as high as 50 mg/l for duration of 10 seconds.
Potassium Permanganate
Permanganate will eliminate anchor worms, fish lice, flukes, ick, cotton mouth, fungus and many bacteria.
The bath containing permanganate should be prepared in distilled water, because a presence of organic matter will lower its efficiency. Potassium Permanganate is an oxidant and waste particles will use up the oxidative energy before potential parasites/bacteria will be affected.
The bath is prepared in a ratio of 2 mg/l for up to 4 hours or 10 mg/L for up to 30 minutes. During the treatment time the solution has to retain the purple discoloration, more permanganate can be added should the discoloration fade before the end of the required treatment time.
Fish should be closely monitored. In case of an overdose, potassium permanganate can be deactivated by an equal amount of hydrogen peroxide. In severe cases, a dip in pure milk will remove all traces of that chemical.