I have tried various things for blanket weed (the green stuff that invades every pond) and have found the best way to keep it clear is simple - use a garden cane, put it into the centre of the blanket weed and twirl it around. You will get loads of weed out and it is actually quite satisfying. Our pond is sparkling clear and you can see right to the bottom of it.
Leave the weed at the side of the pond for a few days to let wildlife get back to the pond, then put it on the compost heap.
Be careful, I once bought out a hapless newt with my cane and had to delicately place him back in the pond - he got a shock, I think he was hibernating!
Lots of oxygenating plants help keep the green stuff down but blanket weed is unavoidable.