Browing Lilies

Some of my lilies are deterioating badly. They are curling upwards at the
edges, even when emerging, drying out and browning/graying (or vice versa).
I have recently fertilized but still have been unable to find Potash. The
pond gets full afternoon sun (area 8b) and a portion of them, but far from
all, do seem to be stiff stems and not receding in the water with
evaporation. Many of the free floating leaves have the same symptoms. Water
is good with about 10 nitrates. ph around 8. Does this sound like too much
directly sunlight (temps low 90's now), nutrition and/or something else.
Thanxx
Bill Brister - Austin, Texas