I’d say the first step would be to find homes for most of the fish. Reduce what’s fouling the water. You can get big totes, net the fish, decide which ones to keep, and put those back. If the fish will be in the totes longer than it takes to catch them, put select ones back, and transport the rest, get air pumps in there. For 2100 gallons, I’d keep 1-2 koi, or 1 koi and some nice goldfish.
One thing I didn’t see anywhere on here, are you on well or city water? I know most of New Mexico has been in a drought, otherwise I’d say do a 30% water change, to dilute the build of ammonia. If you loose any more, check their gills for red coloration. And if you do put them in totes, be sure to use half pond water, half fresh dechlorinated water. Cities add chlorine to waters to inhibit bacteria, but it’s harmful to fish.
Best of luck!