Need filtration system for BIG pond - help!

Beware of Predators! I don't know much about New Jersey but a 2' Koi
pond out here in CA would amount to nothing more than a Seafood buffet
for Racoons. A 4' depth should provide safety from both Racoons and
Herons.

Sad story; recently in the market for a new home. Walked through a
"For Sale by Owner" home in which the owner had built a concrete pond
about 18"-24" deep. She was frustrated because she had repeatedly
stocked it with goldfish, Shubunkins and baby Koi only to find them
all missing the next day.