You have about 470 gallons of water in your pond, if I figured it right, for healthy fish you can have only a certain amount of fish because of ammonia and waste production. (I have just gone thru this quandry this spring and rehomed all of my koi because they grew out of their space in 2 years. They were butterfly koi and the most beautiful fish babies but they could not remain healthy in the pond I had.) See this article- http://splendorkoipond.blogspot.com/2012/03/how-many-fish-can-i-have-in-my-pond.html

The koi will grow quickly and are big waste producers. You will need to buy a kit and test the water for their health. I installed a trickle filter to help with cleaning up their water but it was only adequate for 1 year and could not keep up with their size/waste production. I now have comets, shubunkins and 2 yellow gold fish. They are satisfying my need for color and are very friendly like the koi.

I wish you luck with your fish and know how attached you can get to them and the sadness of having to rehome them. Many other fish lovers on this site will help you with information on caring for your new babies.

I hoped others would give their advice. Do you have any plants in your pond? Maybe some others will respond if you add more info.

I agree with frankielynn, that's a lot of fish for a pond that size. It will support them for a little while but not long so don't get too attached. Keep as much aeration going as you can and make sure to keep the filter clean. You'll find that Koi make a lot of waste and will soil a filter quickly. You should have a water test kit and test your water parameters often. Also make sure your UV (I think that is what you meant when you said 'special light bulb") is working. I would advise that you not feed a lot either. The more you feed the more waste you have to worry about.

Shalom, I have 5 Koi up to 26" and some smaller strangers in a 1000 g or so pond, maybe a bit less; but I run an aereator for about an hour 4-5 times a day as my water fall flow is way too little which when I repipe may help as well. I also, as mentioned in previous posts, use a dirty water pump to keep the pond clean. I also have a 1200G sub pump in the skimmer going into a pressurized filter My centrifical pump won't work because of too much head which may go away when the repipe happens - I also am very heavy on plants in floating islands - you can keep the KOi but it takes more effort