Eyeball it..LOL Nothing scientific about it. I have never checked water PH
etc in over 10 years. It is an outdoor pond . Its conditions are dependent
on how much rain or lack of rain we get. If we get plenty of rain ..that in
itself changes the water in the pond. If it is a dry spell in the Summer I
might do a 1/3 water change every other week. I am not going to drive
myself nuts trying to keep the water conditions at a perfect level. My 10
year old Koi are proof that you don't have to go overboard and make allot of
work for yourself with keeping a healthy pond. It is much better to have
fish that are adaptable to varying conditions, outdoor koi and goldfish are
those types of creatures. Very adaptable. IMHO and experience. MIKE