Ok so is jumping flashing too? And lets say they flash once or twice a day or four times in a day 2 to 3 times a week? I know constantly is too much but after that I am a bit lost as to how much is normal.. Especially if everything else is normal with their behavior. I had this trouble when I got my conure with her sneezing. How much was too much and how much is normal. How often do your goldfish/ koi flash in normal health?
I had something similar this spring and ordered Proform C, but didn't use it as I couldn't quite figure out how extensive a problem it was. Thing is, in spring, this is more common as the fish come out of torpor and the water is still cold, so I waited. Didn't use it, as noted. Your issue though is you have warm water and end of summer temps. From your description, were they my fish, I'd hold off until the frequency is heightened. And yeah, my fish jumped too. I MAY have had a parasite problem and I may not but nature helped, apparently, and I didn't see any flashing since then.
So, I'm not a lot of help, I know, but I'd watch. If they do it constantly and are darting plus jumping, you could apply a parasitical chem, following the directions explicitly. It's either going to get better or worse, as in my case. I didn't want to do such huge water changes and I couldn't easily confirm my gallonage, so I was hesitant without clearer evidence I had an issue. The Proform C wasn't cheap, but I got it just in case and will be ready should I need it.