150 fish will be a problem in the future. I suggest you cull in the future, preferably when the koi get a little bigger.
Here is a quote to live by.
Koi grow, Filters dont.
A filter can not filter everything and support life if there is too much of a bio load in the pond. They say that a filter can filter 50,000 gallons. Which is true and not true at the same time. You cant put a 400 5" koi in a 10,000 gallon pond and expect the filter to handle that much bio load, or even 200. Your filter is going to totally crap out and the ammonia, nitrates, nitrites are all going to go off of the charts, and your koi will die. If by some chance your filters can handle it, when the koi grow to say 8 inches, it is like having 2 4" koi, or 1 3" koi and 1 5" koi. so you said you had 150 5"-6" koi.
That is like having forty 26" koi swimming around your pond.
But say all of your 150 koi grow to 15". Well then the stocks would be much much worse. That would be equivalent to having close to Ninety 26" inch koi swimming around. Now lets add into the factor that they spawn when they are 15 inches, Millions of Eggs will spew into the pond and raise all of the Parameters and cause a massive fish kill, thus you lose all of your koi.
What I want you to understand is that you need to cut down on your koi before they get out of hand. Having 30-40 5"-6" koi would be great in the 15,000 gallon pond that you have. That would allow them to grow quicker and bigger, and you wouldnt have to worry to much about overstocking.
I am really happy I am kicking it off so well with you.! I see some potential in you, you could really become an awesome hobbyist, or even more.
Welcome to the great world of Nishikigoi (Koi)