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If the jumping is occurring without flashing (scratching themselves on pond surfaces), then I'd suspect pH fluctuations. Could be other things, but I'd monitor pH first. You'll need to get your own test kit for it.
....Having the store clerk tell you the conditions are "near perfect" is not very helpful. We'd only be guessing what was checked. It would be best to get your own test kits, but until then, have the folks at the shop write down the test results for you. Are you sure they tested for ammonia/nitrite?
BTW, I stopped using the fertilizer tabs years ago in my lily pond. It became clear that within days the fertilizer leached out of the soil into the water. Sometimes there was an ammonia spike. Other times algae growth went nuts.
It sounds like you have a rock bottom pond. If you search this forum you will find a recent thread about the problems in keeping koi in that design.
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