Help koi deaths

[KoipondAndy]

Been loosing koi since early last year. They stopped dying in the winter due to cold temperature I believe . Have a 30,000 ltr pond with Oase drum filter and shower, 2 oxygen pumps, 2 water pumps & UV.
Water test show 0 Ammonia, 0 NO2, 5mgl NO3, PH 7.5, GH 6,
Done scraps last year and had flukes (which I cleared with lernex) and trichodina which was a lot harder to clear. Used a few treatments (not together) but I believe PP cleared it in the end. I have lost half my koi stock now and have had my first fatality of this year today. I did a scrape and looks like flukes is back (or didn’t quite go). Looks like the fish are dying of bacterial infections with fin and mouth rot. Have done doses of Virkon. I assume I’m doing the right thing treating the parasites. (Also Have Sturgeon). Is it the fluke causing the deaths? They are spaced apart ie every 2/ 3/4 weeks. Anything I’m missing as really feel stuck thanks in advance

[Alburglar]

Very likely that Koi went into winter a bit under the weather due to parasites, so will be suseptibal (is that the right spelling?), to bacterial infection and more parasites now the pond is warming up. You've done well so far, knocking the bacteria back but it will be scrape and treat accordingly once a week til everything is gone. Then do an extra dose of each treatment (according to what you find)for good measure, then think about adding some salt to the pond. Hopefully that will sort it.