The big koy we have eats the algea, but its growth still exceeds that of his hunger. And he's a big fish ~ 20 inches long. When the stringy algea gets too long I drop the water level about a foot and go at it with the garden hose with a high pressure nozzle.

But the single best investment I ever made for the pond was a UV filter. The water is crystal clear year round. It used to look like pea soup in the summer. I've had it for years now and it still works great.

As far as poison goes, we've stopped spraying the junipers for spider mites because it always kills off the insect life in the pond.

There's about 50% ice cover now, the fish have stopped eating till March, and this year I managed to clean the bottom of debris before the ice formed. If I don't do that, the young fish winter under the leaves, where they suffocate due to the anoxic water.