Koi was laying on side at bottom of pond

[Waz]

My Shiro is my problem fish, if there's something wrong with the pond he's the first to show me.

I've seen a number of them flashing over the last day or two, and today he was laying on his side at the bottom of the pond. He got upright once i tried to net him and hasn't done it since but clearly not happy.

It looks like there's a split in his fin and it appears red so i'm thinking bacterial.

I've scraped 5 fish, found nothing to worry about, the scrapes are clean.

PH : 8.4
Ammonia: 0
Nitrite: 0
Nitrate: 18ppm
GH: 10

I've put some chrolarime T in to reduce bacterial levels, and will continue doing this for another 3 days unless anyone else has any suggestions?

Pics should hopefully be included.

Thank you