the Pond Boss® Complete Floating UV Filter with Pump is the only sure way to control the free floating pond algae that turns water green but like pond filters, they must be sized correctly. The flow rate should be at least half the garden pond volume every hour, more if the garden pond is in full sun or contains several large Koi. Once you install the Pond Boss® Floating UV Filter with Pump you will not need to do much more than just sit back and watch your water clear up.