Hospital tank first off, unless he she is alone. Sounds either like columinaris (which is bacterial) or fungus... How exactly does the white stuff look. Is it long and stringy? Is it small compact and fluffy like mold spore fungus?

The great thing about goldfish is that they do well with the old fashioned salt water (aquarium salt not table) cure. From the Dr. Innes book Goldfish varieties:

Salt Treatment.

Most of the disease producing bacteria of fresh water are unable to live in moderate salt solutions. The point then is to find the right strength of solution that will kill the bacteria without injuring the fish.

(treat in shallow water out of bright light when sick)

He uses up to 2 teaspoonfuls to each gallon of water - start with salt mixed in seperate container of same water, and slowly add over time (not all at once) Make sure you keep it filtered (unlike meds can keep charcoal in) and extra airation. Daily change of solution. If no improvement after in four days, gradually increase the strength of salt solution for two or three days until it is up to two ounces (four teaspoonfuls) to each gallon of water. After remaining in this for two days the salt proportion is slowly weakened down again to the first formula....

If you are able to decide what it is and need meds salt will work with them as well (unless otherwise stated).

.....................................

After using many diff meds (and a great deal of money) to treat rescue bettas with the following (water supply during heat wave here) I went back to basic salt water cure, and it made all the difference.

(hopefully others will add to info as without picture or more details???)

Try to describe in more detail if you can? Best Sherry

(by puss do you mean actually oozing some fluid out? Does it look like an ulcer. Or fungussy? Do you see anything that looks like a parasite, little white thread or so? Hole in the head....

Here is a sight for common goldfish diseases (I used it for other non goldfish) it mentions salt in most applications with or without other meds. Melifix I have had no problems with so did you over dose? Make sure you mix all meds with other water fist then dribble into hospital tank...

This has graphics:

http://www.pondkoi.com/pond_products_koi_medications.htm

Re: Columinaris

http://www.aquatic-hobbyist.com/profiles/disease/freshwater/columnaris.html

Here is more. If true fungus (often confused with columinaris) different treatment. note it says may start with wound. (I have also used Iodine on fish wounds and columinaris patches (when it just starts) although I dilute it and use soft cotton swab, be careful not to rub)

You need to go to the site listed below and click on (fungus or hole in head or other) to read full article. Sherry

Fungus

There are many types of fungi and can be broken down more into different categories. I'm just going to talk about the general fungus and that is normally a white cottony looking stuff that will infect an open wound. This again is a bacterial infection and needs immediate attention.

or:

Hole-in-the-head Disease

Small holes appear in the fish's body, usually in the head region. These may gradually develop into tubular eruptions with cream-colored or yellow strings of mucus trailing from them. Fish are lethargic, stop eating and may develop a hollow-bellied appearance. Faeces may become pale and stringy. Lesions may also develop at base of fins and along lateral line, and fins and skin may also erode, body may become milky in appearance and slime coat begins to come off in strands.

http://www.kokosgoldfish.com/disease.html

I think its fin & tail rot, & guess what all my fish have it, in fact the ornge ones tail is just a stub, the rest are missing tail chuncks out, looks like someone bit bites out.... is fin & tail rot contageous????????????