Upgrading filter and water change
New to the site and wanted some recommendations on what to do. So my pond is 3 years old now realized some mistakes thereafter I built it in which I am trying to correct. Pond is about 5000-5500 gallons. About 24 koi, average size is 8 inches. A few goldfish as well that have multiplied each year. Filtration is the savio skimmer, 6000 gph pump leading to the savio waterfall filter. Skimmer has a 50 watt and a 26 watt UV. The pond is filled with 2-3 inch rocks as the bottom. These last three years, water has always been darkish brown and clearing up in the fall to winter. Right now, it's as clear as I wish it could be all year! So I've realized problem 1: no bottom drain. Problem 2: rocks at the bottom. Problem 3: want to remove the goldfish and leave only koi. Pond is 12 feet by 20 feet averaging 2 1/2 feet in depth. I just bought the aqua II 4000 and a 4800 gph pump for the bottom drain. Questions: should I indeed remove the bottom rocks? I'll also have to drain the pond, I live in New Jersey and was planning on doing this first week of April. Would the water change in the spring stress out the fish too much? Or should I wait until May? And lastly, would adding the new filter be enough filtration? Any assistance would be greatly appricieated. Thank you.