Chemistry/Plant help Please!

I posted this one before, but I'll try again.

My Lillies and Water Hyacinths seem to be deteriorating from the inside out.
Looks like they're burning up.

PH seems to be about 8, KH 300+, Total Hardness 75 or so. Nitrates and
Nitrites are nil. Plants did very well last year, WH took over the pond. I
have not added any Potash yet. GF seem OK. Algea is doing well.

Tested all this a couple of more times and checked the local water supply. A
more resonable set of readings would be PH 7.5+. KH 150+ and GH about 75.
The local water authority claims PH of 7.5, KH of 80+ and GH of 141. I've
got a limestone waterfall, so that's why the KH is higher. The strips say
to read them at 25 sevconds, but if I leave them they keep getting darker
and darker. So is 150 KH the real number?

Any thoughts on my "rotting plants", assuming my chemistry is reasonable?
The lillies have two flowers now, but have stopped opening during the day.
Too cool perhaps. I've added a heaping teaspoon of Potash to my 500 Imp
gallons for now.

Any help appreciated.