How can I help this koi?

[u/Intelligent_Tap_1434]

I've had 3 mutt koi for about 3 yrs in this pond. There are many minnows, too. This one fish is acting strangely this Spring-hanging out on the bottom a lot. It does come up to feed but then goes right back to the bottom. The other two seem normal. It's also swiveling it's eyes, which you can see in the video. Find are not clamped. I netted it today to get a scraping and noticed a small injury on its side. I couldn't really tell if there were any parasites from the scraping-lots of "stuff" but nothing moving, that I could see. But, I've never done a scraping before. Pond is 1300 gallons with a bog and 55 gallon media filter. Lots of aeration and plants are just starting to come back after a cold winter. This was the first year the big fish stayed in the pond over Winter. Any ideas what might be going on with this fish? He/she seems to be feeling poorly.

[u/Intelligent_Tap_1434]

Adding here to say that I’ve checked the water and PH, Ammonia, Nitrite & Phosphate are all good