Hi T,
I'm quessing its someplace around the 3,000 gl range give or take. Best if you meter fill it to get exacts should you ever have to treat sick fish.
You really need to know what you have for a pump and filter, very important as you don't want to be under filtered by any means for koi as they are messier then goldy's even!
The rule of thumb for koi is 250 gls each fish minimum. That can be played with a little with good filtration set up and trickle towers etc.
You need to have test kits: Ammonia, Nitrate, Nitrite, PH, GH & KH, DO and salt are good ones also so you can check dissolved oxegen and salt levels. establish the baseline readings with the ph early am, evening, sometime at night to make sure its stabile. And also a ph reading from the water source as well as ammonia.
You will want to invest in a nice QT tank, like a water trough from the feed store. Something that can be set up quickly should you need to deal with a sick fish. Easier to treat a fish in a smaller system then a whole pond if at all possible!
(they also look nice set up with a nice lily in them,lol!)
Find out about any clubs in your area (?) and join as there is a lot you need to learn about Koi and there care!
So where are you? I don't have any baby's yet myself, but who knows, maybe someone will have some. Will help if we know where you are!
I have a pond that I've had running for a month or longer it's 15ft long and 9 ft wide and 3-4 ft deep I have 2 big bunches of lillies cattails irises cannas and someother plaints I don't no the name of and a filter system my pond is clear can any one tell me how many koi that I can put in it . P.S does any one have any koi they need to get rid of don't matter what color I'm not pickey I need some fish. 
