Larry this is very good advise for the beginner pond keeper to follow. But not good advise for the established koi keeper.
Why would I say that? Because the 'philosophy' behind the advise is not the proper perspective.
Here is the contradiction- low alkaline reserve is dangerous in a pond lacking in some parameter or husbandry routine - stocking, feeding technique, circulation rate, filter size and design, maintenance routine, etc. And we associate soft water typically with a low alkaline reserve. BUT koi LIKE neutral'ish' water and they like soft water- the flourishing in such an environment is seen in skin, growth rate and appetite.
I was attempting to lay out to our poster that a dynamic was in place and the steps of that dynamic. You correctly identified the stocking rate. Don correctly sniffed out the likelihood of the pH crash. And the trouble maker Renfield, from the other parish tried his best to embarrass us all with suggestions that somehow we all missed the high ammonia reading. But the truth is, this is all a time line and a cascading event.
You 'could' blame the water itself. A water likely high in carbon dioxide out of the tap ( never did get that intermediary pH reading after aeration) is not 'bad water' . And a neutralish pond is not a bad environment ( in fact it is potentially excellent). It is the other parameters that are testing and straining the situation. And those parameters are - 1) stocking and 2) water changes. A pond stocked to that level and fed well is a threat to that alkaline reserve. Therefore the water changes need to be more frequent and all excess organics must be removed daily. The poster's aeration seems good but I'd have to see the circulation rate and waterfall impact. I'd definitely add a TT to this system. And I'd only stock 8- 10 koi in 2000 depending on size. I would also do a weekly Saturday morning water change in the range of 20% of the water in addition to daily sump cleaning during the feeding seasons. This is now addressing the cart and not the horses behind it!
Hope I made myself understandable here? JR