To be honest with you, I dont know. I did a sorta fish in cycle. I bought beneficial bacteria, had the pond running for two weeks , then added the koi. They have been inside for around 1 and half months. When I noticed the red veins I decded to purchade a liquid test kit. All other fish are fine with no signs of red veins. All fish are eating , and acting normal ect..
I do weekly water changes about 15% and use prime with beneficial bacteria.
That makes it tough not having any idea how recent it was or how severe the ammonia and nitrite spikes were, it can be quite deadly if you add a lot of fish at once to an uncycled system and it can take a few months for the survivors to recover. I do agree with the others it’s probably worth double checking on Koiphen to see if they have any further insight. If you do ever build another pond add one fish at a time (or a couple very small fish at a time), with 4-7 day intervals in between and daily testing. I was able to silently cycle my pond this way (I had the benefit of having run two aquariums prior to the pond).
It’s surprising to me that your pond is newly cycled and the nitrates are already at that level all while you are also doing water changes. This can be a sign of potential overstocking if it keeps building over time. You’ll definitely want to keep an eye on that; more plants may help some.
Generally speaking, weekly water changes really shouldn’t be needed for a pond. I have never done an intentional water change in the past year my pond has been running (there was an accidental one when a disconnected hose partially emptied my pond), and I’m running a very small pond close to what many would consider an overstocking point, but with the bogs and the heavy plantings so far water quality has always been pristine (knock on wood).