erratic swimming and facing downward with tail coming out of the water

[Varty]

Start off with the system. 1300gallons with 10year old koi and added 2 new 8in koi a week ago. Running a Cetus Sieve into a Aqua Ultraviolet Ultima II 2000 Aquarium Filter with a EVO 15watt UV and a Savio compact skimmer. I was doing 20-25% water exchanges every 2 weeks with just the one koi. Besides 8.5 PH all other levels are good(PH is always that high).

Like the title says. 10year old(Blue) is swimming erratic and taking brief pauses facing downwards with tail just barely sticking out of the water. Fins have blood streaking through them. The 2 new koi seem fine. They are hiding from Blue in make shift cover, because Blue was chasing them when 1st introduced. I was told the new koi were quarantined for 30 days before they were sold. Unable to scrape and scope because I don't have access to scope. Any advice would be appreciated!

Video of Blue's behavior
https://www.youtube.com/watch?v=Rpe42EzwuVw

[fly4koi]

Please provide water parameters, namely ammonia, nitrite, nitrate, pH, and KH.

Hypothesis: Assuming water is good, since you added new fish, and old fish start having issues. The fact that it stays near the air bubbles means something is irritating its gill, so I'm guessing parasites.
If you can scope it's the best, otherwise shotgun treatment 3X Proform-C + 2X fluke-M should do it.

If water is bad, obviously use SeaChem safe and water change until water parameter stabilizes and become good.

[coolwon]

Give Blue something to do, allow all three fish to become acquainted.

Let them all go hungry for a couple days, let them meet at the feeding ring.

The younger 8 inch, agile fish will get out of Blues way.

He is 10 years old, and his home has been invaded by others.

Let him loosen up by chasing the intruders.

Wear him down.

Koi are docile social fish, they need company to be happy.

They will shoal and swim together depending on the their mood.

[KingstonKoi]

How are things going now? It’s been three weeks since you originally posted.

When Koi have not had any new fish added for 10 years, there is a syndrome called “old pond syndrome.” New fish, introduce bacteria which normally would not bother other fish. But if a Koi has been isolated in a pond without any new fish for a decade or so, the new fish may challenge its immune system, which has become weak due to a lack of new organisms over a long period.

Kodama Koi Farm has an excellent article on the syndrome at https://www.kodamakoifarm.com/old-po...-death-random/.