You would have to adopt the fish to do this, it is not for the casual "They are just fish" sort of person.
Without knowing the water parameters or tap water parameters here is the most conservative approach:
Test the tank water with all your test kits. This may be a condition called Old Tank Syndrome. porr tank maintenance, few and infrequent water changes, no gravel vacuuming... all lead to a complex of water chemistry that takes some patience to reverse. The fish are somewhat acclimated to the poor conditions, and changing the mineral levels in the water too fast can kill them.
First Week:
10% water changes daily, with as thorough a gravel vac as you can do with each water change. This may not be much at each one, but it will add up. Clean the filter media as needed in the water removed from the tank at least once a week, twice or even 3 times might be needed if this is a small filter and too many fish. Use a dechlorinator that also locks up Ammonia, Nitrite and Nitrate. (Prime, Amquel Plus). If you see some actual disease symptoms then use the appropriate medication, but be very careful to research the medicines. Most are safe at a wide range of typical aquarium conditions, but some are better at a specific pH, or toxic under the wrong conditions. Antibiotics are more likely to kill off any remaining nitrifying bacteria when the conditions are already marginal for them, too. I would also add some salt (sodium chloride) at the rate of 1 teaspoon per 5 gallons each day for a while. Remember to think carefully about how much to add with each water change, then add a little more for that day's dose. Ultimate dose should be about 1 tablespoon per 5 gallons. Goldfish are fine with salt, and it will help if there is any nitrite present. It helps with osmotic stress, too.
Second Week:
20% water changes daily, and continue the gravel vacs. By now the bottom of the tank is getting a lot cleaner. Repeat filter cleaning. Keep up the salt level.
Third Week:
By now the water in the tank is probably getting better, more like the tap water. If it was OTS, then the pH was too low to support nitrifying bacteria. Now the pH is climbing. Add a source of nitrifying bacteria such as some filter media from a healthy tank, or Dr. Tim's One and Only or Tetra Safe Start if needed. (Remove antibiotics or other fish meds before introducing these bacteria) You can also do larger water changes when the tank water is more like the tap water. You might be able to do 50% every other day this week, and may not need to do so many next week.
By the end of the fourth week the fish and bacteria should have recovered as much as they can. Lingering effects of ammonia or nitrite toxicity on the goldfish can be unthriftiness, poor immune system or permanently injured gills. Permanent stunting from being in too small a tank is also common in goldfish. Most people do not realize how large a tank they need, and how the growth hormones and wastes build up in small tanks. Also, by the end of the fourth week you can begin removing the salt. Do this slowly, by adding less and less with each water change for about a month. Then do not add any with the water changes and the salt level will keep going down.
Ultimately the optimum water for goldfish is slightly hard, somewhat alkaline. They handle a wide range of temperature, but thrive with high oxygen levels, so somewhat cooler water is better. I would aim for these parameters:
Temp: low 70s F (low 20s C) (Fancy Golds mid 70s)
GH and KH 5-10 degrees
pH mid 7s (7.0 to 8.0)
Ammonia and nitrite of course, 0 ppm
Nitrate: keep it under 20 ppm.
Water turnover: at least 10 tank volumes per hour, but also make sure there is a calmer area, especially for fancy golds. With their distorted body and fins they are very poor swimmers.
Through all of this I would be feeding a generous rotation of food that includes fresh fruit and lightly cooked vegies, varying sources of protein such as whole fish and shellfish based flakes or pellets, fresh, frozen or freeze dried worms, insects, crustaceans... Each food offers something to their diet. Even foods that some people say are low in nutrition might be high in roughage, or something!
Make up a batch of food based on the recipe here at Loaches, but double the amount and variety of vegetables, adding yams, squash or pumpkin, spinach, crucifers, peas, and especially garlic, and powdered spirulina. Roughly 50% plants based foods and 50% animal based protein sources.
http://www.loaches.com/articles/home-co ... n-fishfood
Good roughage is especially important for fancy golds. They are very susceptible to constipation. Peas and Daphnia are good.
If you have identified a specific disease you may add some medication to a small amount of the home made food and give the fish their antibiotics, wormers or whatever in their food. This is safer for the nitrifying bacteria, and gets the medicine right to the fish, where it can do the most good. Of course the fish still need to be eating with good appetite for this method of medication.