If you have alot of plants take them out, they increase the wrong things if they have alot of dead roots or plant matter. The other major problem I see is you probably have a ton of gunk accumulated on the bottom from food, leaves, and poop. Your filter porbably is not enough biological filtration and you need to pull from a bottom drain to keep the bottom clean. If one is not installed you can either make or buy one the sits inside the pond but sucks from the bottom at it's deepest point.
If you are going to wait until tomorrow, when you can already see your koi are having trouble breathing, then you may well lose some or all of them. Your koi right now are literally being poisoned by their own waste and rotting leaves and food. If I were in your shoes, I would get to the store NOW and run some fresh water in while I was gone, then when I get home I would clean the whole bottom of the pond with a vacuum or siphon. I would also consider a larger water change if they still apear to be struggling to breathe.
After that you need to investigate the different filtration methods available and make sure you have enough oxygenation, and biological filtration. Your stocking density depends on your bio load and biological filtration.
Let us know how things are going. You can go to
www.koichat.com it is a forum based in the UK and the owner can supply you with everything you need and put you in touch with folks who can help in emergencies and local area delaers who can supply in emergencies. Good folks over there. If you really want to learn about filtration visit Maurice's koi farm, his web site is
www.koi-uk.co.uk or go to Yume Koi
www.yumekoi.com , they both advertise on this board. You need to learn more about filtration and water quality so this doesn't happen again. We are glad to have you here, and thanks for being humble enough to share what was really going on so you can find help.
And after all that work is done, sit down chill out by the pond with a nice cool drink in your hand and enjoy it like you are supposed to!