Your diagnosis of a fungus would appear to be correct. The obvious cause being that the fish was outside of the water for some time and it's natural slime coat suffered damage leaving the fish susceptible to the fungus.
To begin to treat this begin by changing 35% (3 1/2 gallons) of the water in the tank using dechlorinated water that is the same temperature as the water in the tank. Right after doing the water change add 2 tablespoons of aquarium salt as the salt will help the healing process and prevent secondary infections.
Then add a product to the water called Stress Coat, click on link below for a view of the product. Stress Coat will help the healing process and aid with the fish's loss of natural slime coat. Stress Coat can be readily found at most major pet stores.
Once all of the above have been done begin to treat the fish with a medicine called
Methylene Blue which is the strongest medicine available to treat fungal problems in tropical fish. Click on link below for a view of the product. Just follow the directions on the product label for dosage amounts. It, like Stress Coat, can be easily found at most major pet stores.
Throughout the treatment process try and do frequent 15% partial water changes as optimal water quality will definitely help speed up the healing process.
Best wishes and please let me know if you have any questions.
http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=4025
http://www.drsfostersmith.com/product/prod_display.cfm?c=3578+4615+4822&pcatid=4822
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Thank you for your quick response. How long should I treat my Koi with the Methylene Blue? Also, I have seen Malachite Green mentioned in the Koi disease web site. Is this similar to Methylene Blue?
Thanks,
Carole
You're welcome Carole.
Listed below is a link which is actually a facts sheet concerning Methylene Blue. It
will supply you with all pertinent information concerning it.
And yes, Malachite Green is also a very good medicine and also treats fish fungus.
This can also be used but I generally prefer to recommend Methylene Blue as I feel it's a bit less tough on the fish and as your Koi is still young I took this into consideration when I suggested the Meth Blue.
http://www.novalek.com/kordon/methylene_blue/index.htm
Thank you for your quick reply. How long should I treat my Koi with Methylene Blue?
As per the facts sheet supplied earlier-
"treatment of fungus or external parasitic protozoans: (a) Remove carbon filter and continue to operate with mechanical filter media throughout the treatment period. (b) Add 1 teaspoon of 2.303% Methylene Blue per 10 gallons of water. This produces a concentration of 3 ppm. Continue the treatment for 3 to 5 days. (c) Make a water change as noted and replace the filter carbon at the conclusion of the treatment."
..
Thank you again. I am beginning treatment. The fellow who sold me the Methylene Blue also suggested I use Tetracyclene capsules in conjunction, one every third day. What is your thought on this?
Tetracycline is basically a broad based antibiotic. The Tetracycline was probably recommended so as to be a compliment to the Meth Blue which is more fungal oriented.
I don't see any harm in using the Tetracycline along with the Meth Blue but don't see it as absolutely necessary. My only concern with using more then one medication is the possibility of overdosing the fish with meds.
So what you can do is start off by adminsitering the two meds at one time and if the fish reacts poorly to the combination immediately do a 25% water change and proceed using only the Meth Blue.
Thanks for all your help.
You're most welcome and good luck on a speedy and full recovery for your koi.