The most effective way to get rid of ﬁlamentous algae is the use of an algicide. Algicide is a weed killer so you need to be very careful using it in a pond containing ﬁsh. If you overdose by 20% with this product you run the chance of killing your ﬁsh. On the other hand, if you know exactly how much water is in the pond and you dose the algicide correctly it can be very safe and work wonders at ridding the pond of this algae. If you are planning to use a herbicide to kill off large amounts of ﬁlamentous algae in your pond always try to remove the bulk of algae by hand ﬁrst because the decaying of the dead algae will consume oxygen from the pond. It is good to always make sure there is plenty of oxygen in the pond for the ﬁsh when you use an algicide.
If you are not sure of the oxygen level consider adding an air stone during the treatment period. Note; Even when used properly, the recommended dose of algae control can be stressful to koi. If ﬁlamentous algae is a problem in your pond we recommend you carefully treat your pond once with a full dose and then a second time, a week later, if needed. Usually two treatments will rid the pond of ﬁlamentous algae. Then consider treating the pond only once every two weeks with half the normal dose as a maintenance dose. This lower dose is much less stressful to the koi and should prevent the need to treat with a full dose again in the future.