UV clarifiers will work on the suspended algae, and hydrogen peroxide will work to keep string algae in check without affecting the fish. the first couple years my pond had trouble with green water, and I used a UV clarifier. things seemed to stabilize after that and I no longer need it. there seems to be a bloom every spring not long after the ice is off, but then no problems the rest of the summer. I still get string algae, but like I said pouring a little peroxide in will help keep it in check.