at those dimensions, boxed.. your pond would be just over 2200gals... enough for 4 or 5 even larger koi...you could sell and replace with smaller fish every few years as well. that depth would really lend itself to your fishes safety from preditors, and water temp stabilty. i keep 5 large koi, and their yearly fry, in just just over 1/2 that volume. good filtration allows me to do so...not that i would reccomend it but, i have 50 fish in 800 gals of water right now, all koi ranging from 3" - 20". 5 over 16" and a 19" pieco. ...regular waterchanges of 5% -10% can do wonders for the health of your pond and fish. larger than 10% water changes i leave for emergencies only. i do a 50 gal water change every 4-5 days on my 800 gal pond. and i do it by flushing my filter, and just replacing the flushed water. this gives my waterchange and filter a cleaning.