Why algae grows when the light is strong

Written 2026-10-02

A strong light itself doesn’t cause algae — the light and nutrients your plants can’t use do. We ran the Water Kiwoogi simulator on a 60cm standard tank with the same high-light LED (light level 3) for 60 days. With no plants at all, the algae index (the 0–100 number the app shows right on screen) climbed to 72.2 by day 60, landing the “Algae Hell” ending. With the same light but a demanding carpet plant, Glossostigma (20 stems), and no CO₂, the tank looked fine at day 30 (algae index 6.7), then the plants slowly weakened, fully melted by day 57, and the algae index jumped to 57 by day 60. The exact same plants with CO₂ added finished day 60 at an algae index of 0.4. All three tanks had identical light — the only difference was whether the plants could actually use it.

It’s not how strong the light is — it’s the light left over

In this simulator, algae doesn’t grow from how strong the light is on its own. It grows from however much light the plants leave unused after photosynthesis, multiplied by how much nitrate and phosphate are around to feed it. With no plants, every bit of light the LED puts out becomes “leftover light.” With healthy, dense plants soaking it up for photosynthesis, there’s that much less left for algae to use. The app’s own note on the high-light LED already says it plainly: “without CO₂, algae hell is practically guaranteed.” That’s exactly why — crank the light up and either skip the plants that could use it, or add plants that can’t get enough CO₂ to actually use it, and the leftover light grows right along with it.

Same high light, three different plant setups over 60 days

Everything else was held constant — a 60cm standard tank, a medium canister filter, the high-light LED for 8 hours a day, 10 neon tetras added on day 1, and a 25% water change every week. Only the plants and CO₂ changed: ① no plants ② 20 stems of Glossostigma with no CO₂ ③ the same 20 stems with CO₂ added. The neon tetras survived all 10 of them in every one of the three tanks for the full 60 days.

Plants can slowly fall apart even without CO₂ killing them outright

Glossostigma is one of the most CO₂-hungry carpet plants in this catalog. Without injected CO₂, dissolved CO₂ sits at the natural background level (around 3ppm), nowhere near enough to make full use of a level-3 light, so the plants lose a little health every hour. That’s why the algae index still looked fine at day 30 (6.7) — the plants were quietly weakening the whole time, and algae kept creeping up underneath (31.1 by day 45, 49.4 by day 55) until day 57, when the log recorded “Glossostigma has fully melted” and the carpet was gone. Ammonia ticked up very briefly as the dead plants decomposed (0.04ppm on the test-kit scale) but was processed back down within a couple of days. With the plants gone, there was nothing left to use the light, so the algae index rose to 57 by day 60. The same plants with CO₂ (setup ③) kept using that light the whole time instead, and the algae index stayed under 1 for all 60 days.

So what should you actually change

If a tank has strong light but only algae seems to grow, there are two ways to go — turn the light down, or give the plants what they need to actually use it. Running without CO₂ is safest with a low- or medium-light LED. If you want high light for carpets or red plants, CO₂ needs to come with it. Once algae has already taken over, reaching for an algaecide first is tempting, but it damages plants along with the algae, and the dead algae decomposing afterward eats into oxygen too — the thing to fix first is the balance between light and plants, not a bottle. Plugging in your own light, plants and CO₂ setup lets you watch how fast algae would actually build up before it happens.

The button below is the tank from this guide — a 60cm standard tank, high-light LED, 20 stems of Glossostigma, no CO₂. Add CO₂ to it and you can watch it turn into outcome ③ for yourself.

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