In article ,
Chris Hogg wrote:

The blanket weed (and any other algae in the pond) is being fed by
nitrate in the water, just as any other green plant needs nitrate
fertiliser. As others have said, this can come from several sources.
One not mentioned is food given to goldfish. ....

No fish. It's a small pond (12' by 4', 2' deep max).

Maybe it's the considerable sludge in the I installed it perhaps 10
years ago,and I've never cleared it out (because I don't like the look
of the liner, and anyway I thought pond sludge was "good"). And also
you need to sort sludge from creatures, if you do clean it out.

useful advice so far though: thanks.

John