2 Koi Ponds. High Nitrates

I visited my dad’s friend’s 2 outdoor ponds today and here are the parameters:
2700gallon pond, 5.2ft
33 large koi, pond 2 years old:
KH 65
GH 20 – 40
Ammonia  0.1
Nitrite 0.1 – 0.3
Nitrate 50+ (under 100)
One fish in the large pond has bulging red eyes, another has a big bulge on the side of its body.
770gallon pond, 3.5ft
20 large koi, pond 6 years old:
KH 30
GH 20-40
Ammonia 0.1
Nitrite 0.1-0.3
Nitrate 50
He appears to have a problem with high nitrates and low KH and low GH. He had to keep doing water changes to keep nitrates low or they can exceed 100ppm. The ponds are quite full with koi and they swim slowly.
Each pond has a small waterfall that brings water down from above and a filtration fixture at the bottom on the pond that sucks debris out to an external canister. The larger pond has a fixture at the bottom of the deep end that can push water upwards with surface action like a pond pump.
There are constant low levels of salt (0.12) in the ponds to prevent infection.
I’ve forwarded this site to Dad’s friend and linked this blog post to him. Hoping he can followup with you here on how to treat.