Hello Veronica . . .
Sorry to hear that you've lost all your koi.
It would appear that you had a classic pH crash where your pond water had insufficient KH (carbonate hardness; also called alkalinity) to buffer (maintain) your pH. When the pH drops rapidly, the koi suffer from acidosis and will die if the balance is not restored.
Here's a bit from Ken Austin (KHA) explaining some of the dynamics:
The understanding of alkalinity (sometimes called kh) requires an understanding of the chemical dynamics of your pond water. When you built your pond, you started out with water, H2O. But, it was not just water. It had some mineral content (mostly calcium, Ca and magnesium, Mg) which showed up when you tested the water for hardness. It also had some carbonates, CO3 and bicarbonates, HCO3, which showed up when you tested the water for alkalinity. Then you added fish and the first thing the fish started doing was consuming the oxygen, O2 from the water and replacing it with carbon dioxide, CO2. At night time, when photosynthesis can not happen, plants are also adding CO2 to the water. CO2 and water do not mix very well. The water molecule wants to breakup. So, H2O + CO2 become bicarbonate, HCO3 plus a free hydrogen ion called hydronium. The hydronium then reacts with the bicarbonate. to form carbonic acid H2C03. In other words, you now have more acid in your pond and that tends to drive the pH down. Don’t worry; remember all that mineral content you have in the water? The calcium, which probably exists as calcium carbonate CaCO2, reacts with the carbonic acid to form bicarbonate HCO3 and some free calcium ions. Other similar reactions will occur with a variety of mineral ions that may exist in your pond. Lo and behold, all that acid that was once in your pond is now removed and there is plenty of bicarbonate, which drives the pH back up to where it started from. Adding any other acid (read: any product labeled as pH Down) will do the same thing. It will temporarily drop the pH, but once the acid is completely reacted, the resulting bicarbonate brings the pH right back up. Does this mean that the acid reaction discussed above will continue producing more and more bicarbonate, which will continue to drive the pH higher and higher? No because bicarbonate has a very stable pH about 8.0 – 8.5. What may drive the pH higher is a hydroxyl ion (one oxygen atom and one hydrogen atom). The abundance of bicarbonate in the water reacts with hydroxyl ions producing water and carbonate. Carbonate, CO3 also has a very stable pH, but it is much higher than bicarbonate. But again you are in luck. Remember all those calcium ions that were released during the carbonic acid reaction above? The carbonate will react with them creating – you guessed it, calcium carbonate. Once again, bicarbonate becomes the dominant organic carbon in the pond and the pH is stabilized at that nice 8.0 – 8.5 range. It is important to have a surplus of bicarbonate in your pond to ensure that all acids and all hydroxyl ions are reacted (this is called buffering). This gets us back to measuring alkalinity. Alkalinity is a measurement of how much bicarbonate and carbonate you have in the water. An alkalinity of 80 – 120 ppm is ideal. Koi can tolerate up to 300 ppm alkalinity. As the alkalinity drops below 80 ppm there is a chance that all carbonate will be reacted with acid causing a significant drop in pH to levels below 7.0. This is called a pH crash. Alkalinity levels below 40 ppm are very risky.
Source:
Alkalinity and KH.. by Ken Austin
What I'd like to see you test to confirm a pH Crash and not some other dynamic going on:
pH from tap AND from pond
KH from tap AND from pond
GH from tap AND from pond