Plants turning brown
 

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. :(

Any thoughts?