Changing water is great. I don't know that I would be using any softened water as an increase in both KH and GH are probably needed to bring the pH back to stability. Check the pH late in the afternoon, early evening and if it is over 8, add baking soda to bring the KH up to over 150 by using the calculator at the top of the page to determine amount needed. Due to diurnal changes in pH when the KH is low, the pH should be its highest at about sunset, and the shock of increased pH by the addition of baking soda will be its lowest. The filter is suffering a pH crash causing the ammonia reading and the nitrite reading, but if the pH is stabilized, it should return to zeros soon. In the mean time treat the pond with a complete pond dose of SeaChem Safe, Prime, Cloram-X or other treatment for chloramine to bind the ammonia, and help with the nitrites. The burns are probably associated with the pH changes due to the pH crash, and the effect may be temporary. Get the water right, NOW, and keep an eye on the fish.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard