Help Pink Fin

I posted a few days ago about some abnormal behavior around the waterfall that I was noticing with my Koi. Fast forward, and most of the baby Koi hybrids that were in the pond are all dead and now my large butterfly has pink fin. Clearly there was a big issue affecting all of them. There’s definitely overcrowding because I had all of these babies in here that I was trying to get out. That situation solved itself, but now I’m dealing with sick fish and I don’t know what to do.

Has anyone tried putting some of the bacterial pond stuff just straight in the pond? Any other advice would be helpful