How to Kill Green Hair Algae in a Freshwater Aquarium
To quickly kill green hair algae in an aquarium, you can spot treat with 3% Hydrogen peroxide, around 1mL per 10g water volume. Turn off the filters, and fill a syringe with the peroxide. Spray directly on the algae. Leave the filters off for about 15 minutes, do a 25% water change, and turn the filters back on. You should notice the algae starting to turn reddish or brown by the next day. Repeat as necessary.