That sounds like a good idea, over cleaning could may well be the issue, I would suggest seachem prime or seachem amguard, it will help will converting ammonia to ammonium which will help a lot.
Sounds like ich especially if you can see the withe spots with that being said I’m sure fluke worms have similar symptoms minus the white spot @Colin_T will be able to help more.
Yes that’s your best bet there are a lot of people here that will help with that, before you know it your pond will be cycled and you’ll be adding your koi.
It’s not the bio load, cycling with fish will induce stress when toxins rise and fall further causing diseases like ich. Treating that volume of water for something like ich will be costly also all that can be avoided if you do a fishless cycle.
Honestly I would say cycle first since it’s a 1300 gallon pond coz if there are ammonia and nitrite spikes doing water changes to get things under control would be horrendous even worse if you have to do more than one water change in a day. Think it over 1300 gallons is a lot of water.
Ideally you would want to house them in something larger so that they are comfortable since 3 weeks is a long time just to reduce stress as stress can cause diseases just as much as poor water quality can.
good luck!
filtration x4 turnover
With water change you want to test the water if nitrates or any toxins are high you would want to do 50% water change then test water repeat water change if test results show any toxins or if nitrates get too high.
once introduced to the pond realistically you would want...
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