Eeek! I don't know that you will ever know for certain if it was the wound that killed it or the disease.
Fish will attack each other, even nonagressive species will do this occassionally. Looks like a small wound that became infected; absessed, festered and grew - that probably left the fish weak and therefore open to waterborn pathogens and parasites.
Do your other fish show symptoms of ich or fungus? These pathogens are normally present in all water enviroments to varying degrees, but a healthy fish can ward these off with their own immune systems. Once the fish is stressed (fluctuation in PH, dramatic change in temperature, injury or any sort of stress) can make the fish more vulnerable to them.
I'm banking on the injury killing the fish, and the parasite or fungus simply taking advantage of an already weakened fish.

It appears to be a bacterial infection and that will easily kill the fish. I can't tell from the pictures what the white is but if I had to guess I would say excess slime coat. Yes the infection could have started from a parasite or fungus or nothing at all. This is a new pond and fish get infections in new ponds easier than a older pond.
Mike

destany,
My chagoi is showing one little spot (maybe - it hard to tell - she's strong and may have nick it climbing through the parrot feather the other day on her "love run". I added a pond treatment and quit feeding to allow the water to clear / nasties to (excuse the pun) die down.

Mike_il,

Odds on the culprit was bacterial. Brought on by excessive green water: probably brought on by my introduction of fertilizer tabs to my lily / lotus / plant tubs and not placing them deep into the soil; probably releasing nitrogen into water; Brought on by me, the newbie, placing excessive food into water.. to get the fish up out of the pea soup, so I can see them. You get the idea...
{{gwi:190885}}

Now for you Mike_il - a follow-up question. I seriously looking at purchasing a UV unit (Emperor Aquatics SMART UV 65 watt Wiper). It states that it can handle a max pond size is 8,565 gallons. My pond is 6000 gallons/ full sun / southern CA. Is this unit big enough to meet my needs? I heard people mention you should have a UV unit twice your pond size for the unit to be effective. Your thoughts please.

LP

Some of my goldfish got something similar on them. One died before I could treat them. They had yucky fuzzy looking stuff on them. The second one was sort of floating a lot, so was pretty easy to catch. I used TriCide Neo Dip with an airstone and dipped him everyday for about 4 days. I also fed Medi Koi fish food for 14 days. As I was fixing to start re-building my pond. I dipped each one in the dip before putting them into the temp holding pond. The one that was really bad, looked like someone had taken a pair of tweezers and pulled off some of his skin, after the fuzzy looking stuff fell off. Sort of like yours looked. But mine didn't look as raw and everyday it looked better and he has recovered now. Now all my fish are looking good and enjoying their new home. Glenda

you probably have a few things going on. Since the koi lived long enough to get a nasty infection, we can pretty well rule out parasites such as costia and ich since those can kill pretty quickly. I suspect a parasite was still responsible, possibly flukes. Flukes are irritating and cause a break in the skin. The bite gets infected, causing an ulcer. The fish is now sick, and a fungus called saprolegnia starts growing on the ulcer. Saprolegnia releases toxins into the fish making him even sicker. SAP alone can be deadly. I realize it is too late for this one, but in the future here's how I would treat that fish.

Catch the fish and sedate. Gently scrub the wound with a q-tip and betadine. Remove all traces of fungus or it will grow back. Be sure to keep betadine out of the gills!! Treat the sore with a paste made of potassium permanganate (dangerous stuff....handle with care), or malachite green. Seal the wound with denture powder. With an ifection as bad as that one, I would also treat with an injectible antibiotic such as amikacin. The fish needs to be returned to a clean QT and the temperature needs to be slowly brought up from the pond temp to around 70°. A koi cannot heal in cold water.

Further investigate the possibility of flukes on other fish. Symptons include flashing, jumping, excessive slime coat.

LP It was not green water that caused the infection and the fish will actually do better in green water. Green water can drop the O2 levels so low as to kill the fish and can raise the O2 levels so high to give fish a problem. In order for a fish to get an infection three things must occur at the same time. OThe first is the fish must be stressed or immune system not working. The cold water will do this. Second there must be something wrong with the water quality. Again coming out of winter or the fertilizier could cause this. And than a break in the slime coat. This could be a parisite or scraping themselves on a rock. I would agree with ccoombs that it probably isn't costa as that will usually kill fish in cold water. Ick usually won't kill koi, goldfish yes but not koi. It could be anyone of about 10 different parasites or more than one. The recommended treatment is excellant if the fish has a fungus and if it doesn't I would everything else.

The Emperior 65 watt UV will handle your pond just fine. I am not crazy about the 65 watt just because it is so long. I would prefer the 80 watt but that is just personnal taste.
Mike

Sorry folks, I was down south for a few days.

Came back to a little more clarity in the water, but still a green bottom. Fish look active and no change in color / appearance. I'll do another fungus / pond treatment for insurance.

mike_il,
Yeah I agree on the 65 watt's UV length. I looked at the 65's numbers and commented: "How the $@G^& does one "logically" put that monster into line with their system?. I too (after being knock upside the head by a more knowledgeable person) like the layout of that particular unit.

My water temp has been doing roller coasters the last couple of weeks. For example, March 12 (72 deg. / bottom 70) March 18 (62 / 60) March 24 (72 / 70)April 8 (64 / 62 - fish died). It was right after that 62 that my water clarity went south... also around the time I added plant fert. Could this be a major factor in my fish demise?

Next question, my NH3 / NH4 levels have been unfortunately holding at 0.25ppm. My PH has been 7,Nitrate/Nitrite levels 0 the whole time. I know a reading of 0ppm is ideal, but can I hold this level(working towards lowering it) or should I use an ammonia blocker ASAP?

Thanks for the info.

LP

the other night we had a horrible thunder storm with pouring rain ,the next morning my large koi were all dead. The goldfish was fine. I have had the koi for 10 years and no problems. Thank you