20/25g of PP is far to much and as you have used so many treatments that is more than likly what has caused it and as for the fish's gills a lot of the treatments you have used especialy the PP will strip the oxygen out of the pond and with no oxygen in the pond the gills will start to deteriate, as said above you need to get the water quality spot on now i would do some 10% water changes of the next couple of days adding filter boost to get the filter back on track and add NO MORE TREATMENT get your water perfect and then if the fish are still looking ill get a scrape done again and go from there but you need to give the treatments a chance to work. I know its hard at the min but i went throu this at the begining of the year with PP striped the oxygen out of the pond and with in a few hours i lost 5 koi so ill never use it again only to disinfect. Just bear with it you will get it sorted
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Hi im Dan and im addicted to Koi | 
