How large are the Koi and the 4 remaining Goldies?

The koi are about seven, eight inches long and the other goldfish range from five to seven inches long.

You may not have done anything wrong. Sometimes the best of our observations (bacterial, parasitic...) aren't enough to determine root cause. As a former coworker once suggested, unless we take some sort of scraping from the cells and put it under a microscope, we'll never know. Now if the others die, one by one, with no apparent external symptoms, I would consider the possibility of a virus.

I've heard similar things regarding fish diseases and identification. I do hope that this is something that does not pass on to my other fish. I've been keeping a close watch on them, and haven't noticed any irregular behavior or physical indicators of a problem. Thank you. I'll keep posted if anything more comes up!

I am sorry, but 2 koi and 4 goldfish in 55 gallons is asking for trouble; especially if the koi are around 8 inches. That is a huge bioload for the filter to handle and just setting the fish up for stress -which leads way to infections.

I agree with bradarmi, and why I asked about the size of the fish. A 55 might do for baby koi or goldies, 1 - 2 inch or so, but if you add up the total length of all the remaining fish, then factor in their bulk, you are keeping the equvilant of at least 500 guppies in that tank, if not more.

One of the problems many home fish keepers create for themselves is, I think, due to seeing fish in pet stores being kept in certain sized tanks and think that's what they can use too, but don't consider the huge, flow through filtration systems hidden behind the display tanks, many of which have automatic top-offs connected to them, that continually adds fresh water.

So if you can't bring yourself to getting rid of your excess fish and can't get them into a larger tank, you can at least do small daily water changes. Just 10% per day is better than a 50% once a week.