I know it will be hard for you to answer this without all the details and specs of my pond, but maybe if you've had a similar experience it will ring a bell with you. I have a 4000 gallon pond in it's fourth year, and everything is wonderful. Clear water, healthy fish, spawning, hatching, lily buds, etc.

Within the past week I have added some potash and some baking soda, but not on the same day. Today I noticed that a LOT of the plants leaves, (hyacinth, lily pads) have lots of big brown patches on them, even the lily pads that are still UNDER the water. Did one of the two additives I put in do this or is it just coincidence? I can't imagine either of them causing that kind of damage.
Sue