"Algae Destroyer Advanced". I had green water for months in my 5 gal. that I couldn't seem to get rid of. 4-5 drops of that stuff and the water was crystal clear in about 2 hours.
I had green water for a while in my 5 gal. So bad you could hardly see the 1 cory cat and 1 Rasbora. Went out yesterday and bought "Algae Destroyer advanced". I did a 50% water change then added 5 drops to the tank. Several hours later the water was crystal clear and is the same today! It calls...
I put the Biobag beads on top of the sponge as well. Never had spill over but that's on a 30. Tilt the filter forward just in case so that overflow (if it happens) goes in the tank. LOVE AC filters! Mine are 5+ years old now.
NO! Try Frogbit! Much bigger than Duckweed and easier to control. I got some Frogbit last Friday (my first Frogbit!) and LOVE IT! Duckweed I have found is a pain to grow, I have some in some plastics tubs and I'm trying to grow some to feed my Red Cap Oranda. But Frogbit is doing well so far.