An ultraviolet light. I have a 15x25 pond, 4' deep at one end, 3' in the middle, and 1' at the shallow end. My pump is 2500 gph with two streams and a series of waterfalls in each.
The UV light kills all green algae and any bad bacteria, and the water stays crystal clear right to the 4' bottom. I still get string algae in the streams and the fuzzy algae that sticks to the side, which is good for the pond but no pea soup.
These lights come in different sizes. There's a black plastic housing that the water flows through. Inside the housing is a long glass tube that the light bulb slides into. Doesn't use much juice, so it's not expensive to run. I leave mine on all year, but, I suppose you could just use it when you get green algae.
It's the best money I've spent for the results I get.
I've tried the barley and the chems, and at least for my pond, it was only a temporary fix that lasted a few days.