Sick Koi: pink / red between scales

Hi everyone

We recently took over a koi pond with two koi. in our apartment block from two inactive owners. After cleaning it and installing plants (in river sand) we noticed one of our goldfish (we have a lot) had mould. Consequently we dosed the pond with PondMedic methylene blue (http://pondmedic.co.za/products/medication/parasite-clear/). Before dosing it the second time we had the water tested and were told it's the best quality water the koi shop owner had seen in a year.

We decided to celebrate by buying a third koi. When introducing it to the pond we didn't quarantine now realize we should have. but didn’t do it as we don't have the means. However, over the next week we did do another two doses of pond medic. Today we noticed that one koi has pink between its scales (see attached photos) and on its fins and is very sluggish. We observed him flashing, yesterday. He's been sluggish for a few days now, and tends to only sit at the bottom of the pond, nose against wall. No appetite for a few days now. We are very concerned. His side fins look a little ragged too? Noticed the other koi (not new) has become sluggish, too.

Yesterday we topped up our pond with about 50L of collected rainwater into our 2500L pond.

We're currently in a drought and don't have access to groundwater so can't do a major water change, only top-ups of about 5-10% at a time.

Anyone have any advice for us?

We we don’t want our fish to be in any discomfort or pain.

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