For over 8 years I have had a pond in the corner of my home. It is of a triangular shape, about 10' x12' x15' (3 x 3.6 x 4.5 m) by two feet (60 cm) deep. For years I have had two koi, random other pond fish and some carnival won goldfish. The last three years or so, frogs have found a home there, too. I have a waterfall and a couple plants.
I've never had any problems and I do very minimal maintenance with no issues.
About a week ago, a fish or two died. For the next couple days, I lost two to three a day. On the third day, I had water tested and it was fine. Next day, more dead fish. I drained 2/3 of the water and refilled, also added stress coat/chlorine reducer from pet store. Next day, more dead. One koi was about seven inches (18 cm) and he held out long, but finally died, leaving just the one koi left. Two days later, that 12 inch (30 cm) koi died.
When no fish were left, suddenly the water got extremely clear. I saw nothing wrong with the fish, except they showed signs of a lack of oxygen. They would float like dead till you attempt to net them, then they would swim. No spots or visible redness was to be seen around gills. Water has been pumped into a fountain the same all year, so why would O2 levels suddenly drop within a week enough to kill all the fish? Water was not warm.