I am converting a irregular 22 X 40 foot gunite pool to a pond. I will be filling it to approx. 3' throughout which comes to about 17500 gal.? I will have two bottom drains with 4" piping and two Savio skimmers with pumps and filters. I still do not know what size primary filter to use, I have been looking at the Nexus 300 with gravity feed, what do you recommend?
17.5 K gallons is a gracious plenty, and the piping, at 4-inch diameter is appropriate. Your pumps and filters should be capable of moving and accepting a flow rate sufficient to send about one half of your total water volume through your filters every hour. Smaller ponds require flows able to exchange the entire pond volume in that same time. Select your pumps and filters with that in mind.
Do not expect your water to be gin clear if you are keeping koi. Koi love gunite ponds, mostly because all sorts of tasty critters burrow in gunite and they can dig happily after them. Your water will reflect this happy activity and will be constantly turbid, since koi eat constantly, given the chance.