goldfish with a fuzzy spot

Question
I have three goldfish and an algae eater that have shared a 30 gallon aquarium for the past 5 years or so.  Everything has been fine until today when I noticed that the largest goldfish has a cottony looking spot, about the size of one scale.  I'm thinking fungus, and I'm thinking beneficial bacteria before any chemical/medicinal antidote.  The fish doesn't seem bothered.  Looking for input....

Answer
Dear Christine,
What you are seeing could certainly be fungus. But I don't like to make any diagnosis right away. Maybe the best option is to view some pictures of other fish that have fungus. Here is a link to some-
"http://www.fishpalace.org/Disease.html#Bodyfungus"
 And
"http://www.thefishworld.co.uk/id62.htm"

Fungus tends to attack injury sites on a fish, and this can happen especially fast if the aquarium has buildup of organic debri.
Before doing any medicating, please do a 30-50% water change and vacuum the gravel well with the siphon. Remember that organics can reduce the effect of medication. ~I would probably medicate with Pimafix which is an all-natural anti-fungal medication that is literally safe for all fish and tends to not do as much harm to the beneficial bacteria (as you were worried about) Even better is Melafix and Pimafix combination which can be safely mixed together to provide extra anti-bacterial/anti-fungal. But feel free to use just the Pimafix.
Along with the medication, please try to do very frequent water changes, at least every to every other day will greatly help. Goldfish are big polluters and three of them (along with the algae eater--maybe a pleco?) will put quite a bioload on your 30gal.
Remember to remove any charcoal from your filter before medicating also.

When caught early, fungus should be very easy to treat and a improvement on tank maintenance should keep it gone for good.

I hope this helps and feel free to write with anymore concerns..

Best of luck,
Karen~