120 gallons is tricky.
The bigger the pond the more forgiving the system.
What do you know about the water you are putting in? Chloramines stick around
much longer than chlorine.
Do you have any plants?
Get a pond test kit and post your readings here for the experts (meaning not
moi ;-)
Also, just go with goldfish. And keep your stocking low. 20 gallons per fish.
Current wisdom is koi need at least 1,000 gallons just to start with.

good luck!
k30a