Why Floating Plants Keep Algae Down

Written 2026-11-02

Floating plants do two things at once — ① they float on top and catch all the light themselves, while blocking the light that would otherwise reach the algae below, and ② they pull nitrate out of the water through their roots, starving algae of a nutrient it needs. We ran the Water Kiwoogi simulator on a 60cm standard tank with a high-output LED and 10 neon tetras for 60 days. With no plants at all, the algae index (0–100) climbed to 72.2, landing in "algae hell." With 8 duckweed plants floating on the same tank, the algae index stayed at 0 for all 60 days, and peak nitrate dropped from 25.2ppm to 7.3ppm on a test-kit scale.

Two jobs only a floating plant can do

In this simulator every plant carries two published numbers — how much light it blocks, and how much nitrate it pulls out per plant per day. Floating plants score far higher on both than rooted ones. Duckweed blocks 0.55 of the light (maxed out once there are about a dozen plants) and removes 7.5mg of nitrate-nitrogen per plant per day. A common rooted plant like anubias blocks nothing below it (it’s underwater) and removes only 0.8mg a day; java fern removes 0.4mg — so one duckweed plant pulls out as much nitrate as about eighteen java ferns. Floating plants also draw CO₂ straight from the air above the surface, so they grow well even when dissolved CO₂ is low, and they get full light with none of the depth loss a tank’s water causes. They sit in the best spot and grow the fastest.

Same 60cm tank, only the duckweed count changed

Everything else was held constant — a 60cm standard tank, a medium canister filter, a high-output LED for 8 hours a day, 10 neon tetras added on day 1, a 25% water change every week, and no CO₂. The only thing that changed was the duckweed count — ① none ② 3 plants ③ 8 plants (enough to cover a good part of the surface). All 10 neon tetras survived the full 60 days in every version.

Even 3 plants cut the algae index nearly in half, and 8 plants — covering a large share of the surface — removed enough light and nutrient that algae growth couldn’t outrun its own natural decay, so it stayed at 0 the whole time. It isn’t an on/off switch; algae growth slows in proportion to how much light and nutrient are taken away, and once it slows past a certain point, decay simply wins.

Nitrate drops along with it

Duckweed doesn’t so much "trap" algae as cut both things algae needs — light and nutrient — at the same time. Nitrate is a separate fuel for algae growth, independent of light intensity, and duckweed pulling it out around the clock means less stays in the water. The peak nitrate numbers above — 25.2ppm, then 15.4ppm, then 7.3ppm as duckweed count went up — are the proof. If you want to know why nitrate keeps climbing in the first place, our other guide on rising nitrate covers that.

A less invasive species works too, just more gently

Our own catalog already warns that duckweed "multiplies explosively, can cover the whole surface, and is almost impossible to remove once it’s in." Swapping in 6 Amazon frogbit plants under the same conditions only brought the day-60 algae index down to 4.2 (peak nitrate 10.8ppm) — weaker than 8 duckweed plants, but frogbit is far easier to scoop out by hand, so it’s easier to keep the amount under control. The flip side is real too: more floating plants means less light for anything planted below, and our frogbit catalog entry itself notes it "blocks light and can kill plants underneath." If you’re also growing rooted plants, it’s safer to keep floating plants under about half the surface.

The button below runs the 60cm standard tank · high-output LED · 8 duckweed plants tank from this guide. Try raising or lowering the count yourself to see how the algae index changes.

The numbers in this guide come from the Water Kiwoogi simulator model; real tanks can differ.