Algae are single-celled or filamentous plants and they need exactly what your lettuce needs: water, dissolved minerals and light. A hydroponic system supplies the first two in abundance, so light is the only lever you actually control. That is why the fight against algae is won with darkness, not with chemistry.
How to tell you have algae
The first sign is not green water — it is a slimy film on the wall. Run a finger along the inside of the tank: if it feels slippery, algae are already established, you just cannot see them yet.
- Green or brownish coating on the inner tank wall and lid — the earliest signal.
- A slimy layer on the roots that will not rinse off under lukewarm water.
- The pump runs louder or weaker because its filter is clogging.
- Tiny black flies that hop around the plant sites rather than fly.
- pH drifts up 0.3–0.8 units over the course of a day with no input from you.
Why algae do real damage
Oxygen
During the day algae produce oxygen; at night they consume it, like any plant. In a tank at 25 °C dissolved oxygen easily drops below 5 mg/L, while roots want 6–8 mg/L. Below 4 mg/L you open the door to Pythium, the organism behind root rot. Dead algae are worse still: the bacteria decomposing them pull even more oxygen out of the water.
Nutrients
Algae eat from the same bowl. Iron, phosphorus and manganese go first, because they are present in the smallest amounts anyway. Typical picture: your EC reading looks fine, yet the young leaves yellow between the veins.
Pump and fungus gnats
Slime collects on the pump filter and inside the narrow openings of the distributor. On a vertical tower that means flow drops and the top rows run dry. On top of that, a damp algae film is the perfect place for fungus gnats (Bradysia) to lay eggs. The larvae start on algae — once that is gone, they move on to root hairs.
Prevention: remove the light, not the algae
| Spot in the system | Why it is risky | Fix |
|---|---|---|
| Tank | Clear and white plastic both transmit light | Opaque dark tank, or two layers of light-proof film |
| Tubing | Clear tubing is a miniature greenhouse | Black tubing, or wrap it in electrical tape |
| Lid | A 2 mm gap is enough for a green rim | A lid that seats properly, refill port with a plug |
| Empty plant sites | A 5 cm hole shines straight into the solution | A cap on every unused site |
| Substrate surface | Wet rockwool in light turns green in 4–7 days | Thin layer of clay pebbles or a black collar around the seedling |
The second lever is temperature. At 18–20 °C algae grow slowly; above 24 °C they explode. In summer keep the tank in shade and never leave your top-up canister in the sun — it usually greens up before the system does.
Removal: mechanical first, peroxide second
Chemistry cannot penetrate a thick layer of slime. The order is always the same:
- Drain the tank. The old solution is great for houseplants in soil.
- Scrub the walls with a soft brush and warm water. Dissolve limescale with vinegar diluted 1:5, then rinse thoroughly.
- Take the pump filter and impeller apart and rinse them — that is where most of the slime sits.
- Rinse the roots gently with lukewarm water, no scrubbing.
- Refill with fresh water, add nutrients, and only then the peroxide.
Dosing 3 % hydrogen peroxide
| Purpose | Per 10 L | 40 L tank | 50 L tank | Frequency |
|---|---|---|---|---|
| Preventive, plants in place | 10 ml | 40 ml | 65 ml | Once a week, after topping up |
| Visible algae, plants in place | 20–30 ml | 80–120 ml | 130–195 ml | One-off, check again after 3 days |
| Flush of an empty system | 100 ml | 400 ml | 650 ml | Circulate 20–30 min, then rinse twice |
Hydrogen peroxide breaks down into water and oxygen. In a nutrient solution its half-life is a few hours — with warm water and a high organic load, under an hour. That means no residue in your vegetables, but also no lasting protection.
- ⚠️ Always add peroxide to a filled tank, never the other way round.
- ⚠️ Gloves and eye protection. Pharmacy-grade 3 % is mild; 12 % and 35 % leave white chemical burns on skin.
- ⚠️ Do not mix with organic fertilisers, humic extracts or beneficial cultures (Trichoderma, Bacillus) — it oxidises them just as fast as algae.
- ⚠️ Never combine with bleach or any chlorine-based cleaner.
- ⚠️ After a 10 ml/L flush, rinse the system twice with clean water before the plants go back in.
Honestly: zero algae is not a thing
A thin green rim around a planting site that stays wet and lit is normal and harms nothing. The goal is not sterility, it is keeping algae out of the tank and the pump. If you find yourself dosing peroxide every single week, light is getting in somewhere — hunt for the gap, not for a stronger dose.
More on the water you fill the system with is in Water quality, and the step-by-step cleaning routine is in Tower maintenance.