Sorry - a bit long winded.
I am not a professional so you may need to seek other advice but I have kept a pond for a number of years and still maintain coldwater fish predominantly koi. I have only relatively recently gone into tropical fish. (8 months or so)
Fortunately none of my tropicals have suffered from this ailment but from experience of koi, from what you describe it could be that they have internal parasites. One treatment for this is to give them live food although that in itself may be suspect to introducing more problems. Only use foods from a reputable retailer if you choose this option.
The second option is a little more drastic but works well with coldwaters.
You need to isolate into another tank all the fish that you feel are suffering or may be infected and gradually increase the waters salt levels to 3 or 4 times normal (upto 2 teaspoons/galon). This serves 2 purposes. One is that it will reduce stress and secondly will help replace the fishes electolytes quickly that they may be losing due to the ailment. In order to raise the levels quickly you will need to do a 25% water change daily each time upping the salt levels until you reach 2 teaspoons/gallon over 3 or 4 days.
One of two things will happen, it will either help the fish recover on its own whilst also oversalting the parasites so that they are killed off and get passed out of the fish naturally, or the fish itself may die if it is not strong enough.
Once the fish is healthy again reverse the salting process over the next 3 or 4 daily water changes, reduce the salt back down to normal levels and return the fish back.
I did say that the second option sounds drastic but as I said it works for coldwater fish.
Maybe a moderator can read this and either blast me or agree with me before you do anything.
Good luck with your fish.
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