Pond goldfish - what is this white peeling skin problem?

[kryten22uk]

Some of my fish are dying. They have this weird problem which makes it look like their skin is peeling like when we get sunburnt. Only noticed it two days ago, so it is rapid and rapid deaths. Tested water and pH is a bit high, but ammonia and nitrates fine. I've put water conditioner in and also put in some fungus treatment (see pic), which by the way stains skin like crazy!