And in my experience pond plants are not so easy to kill. I assume *you're*
talking about plants that need acidic to neutral water? Please clarify.

A great majority of plants will die with pH over 9, where goldfish will
thrive. The poster didn't say what kinds of plants, nor how quickly they
died. It could very well be, as you surmised from the beginning, that the
problem is simply winter. We don't know if Pat doesn't tell us. ~ jan