This is a question only you can answer (honestly)
How many of you test your pond water pH and how often?
There is evidence this year a lot of ponds pH have been slipping into the acidic danger zone i,e, below pH 7. Not only is this bad for your koi's health and growth, it does not help filters maintain their biological action, and also can inhibit them from maturing.
A mature pond and filter system will eventually start to go more acidic, due to nitric acid being produced from the nitrogen cycle, once all the carbonates have been used up, the pH will crash and this can happen overnight!!!!.
So check your pH soon.
Dave.