Green film on the reservoir walls, green fuzz on the clay pebbles, water the colour of weak tea. Algae is the most visible problem in hydroponics and, on its own, one of the least dangerous. The trouble is what it leads to.
Algae needs three things. Remove one.
Algae requires light, water, and dissolved nutrient. You are committed to two of those, which makes this the rare hydroponic problem with a genuinely complete solution: block the light and the algae cannot exist.
Every treatment below is a way of doing that, or of cleaning up after failing to.
Is it actually hurting your plants?
Honestly, a light film usually is not. Plants outcompete algae for nutrient easily, and the quantities algae consumes in a home system are trivial. If you have a faint green tinge on a tote wall, you can ignore it for now.
What makes it worth fixing is the chain it starts:
- Oxygen competition at night. Algae photosynthesises under your lights but respires in the dark, consuming dissolved oxygen from the same water your roots depend on. A heavy bloom can measurably drop overnight DO.
- Decomposition. Algae dies in cycles. Decaying algae feeds bacteria, and that bacterial bloom consumes more oxygen still. This is the actual mechanism by which algae leads to root rot.
- Fungus gnats and shore flies. Algae on damp media is what they lay eggs in. An algae problem becomes a pest problem within a couple of weeks.
- Clogging. In NFT channels and drippers, algae mats block flow, and the first sign is a dry spot in a channel you thought was running.
So: not an emergency, but not something to live with either.
Where the light is getting in
Work through these in order. In most home systems it is number two.
- Translucent reservoirs. White and clear storage totes pass far more light than they look like they do. If you can see a shadow through the wall, so can algae.
- The gap around net pots. The single most common entry point. A two-inch net pot in a three-inch hole leaves a ring of lit water.
- Exposed growing media. The top layer of clay pebbles or rockwool stays damp and lit, which is exactly the condition algae wants.
- Lids that do not seal. Warped tote lids, unused net pot holes, and cable pass-throughs.
- Top-fill and inspection ports left open between uses.
Clearing an existing bloom
- Dump and scrub. Algae lives as a biofilm on surfaces, so rinsing achieves very little. Physically scrub the walls, lid underside, net pots, tubing, and air stones.
- Treat with hydrogen peroxide if the system cannot be fully stripped. 3% household peroxide at roughly 2 to 3ml per litre kills algae on contact and breaks down within a day. It will also kill any beneficial bacteria you are running, so decide which strategy you are on first.
- Replace badly fouled media. Clay pebbles can be washed and reused. Rockwool that has gone green should be replaced.
- Fix the light leak in the same session, or you will do all of this again in three weeks.
Do not use aquarium algaecides. They are formulated for fish tanks, not for plants you intend to eat, and several are phytotoxic at the doses that actually work.
Keeping it out permanently
- Use opaque reservoirs. Black is ideal. If you already own translucent totes, wrap them in reflective film with the white side out so you are not also heating the water.
- Fit net pot collars. Neoprene collars close the gap and cost very little. Cut foil or coroplast discs if you would rather not buy them.
- Cover exposed media with a shallow layer of dry pebbles, or a light-blocking disc.
- Keep water below the media line. In DWC, once roots are established, an air gap between the water and the underside of the net pot means the lit media surface stays dry.
- Tape spare holes shut. Every unused net pot hole is a light source.
Cooler water helps too. Algae grows faster in warm reservoirs, which is one more reason the 65 to 68F target keeps turning up in every hydroponic troubleshooting guide, this one included.
If it keeps coming back
Recurring algae after a full cleanout means you have not actually found the light leak. Turn the room lights off, put a bright torch outside the reservoir, and look for glow through the walls and around the fittings. It is almost always the net pot gap, and it is almost always more light than people expect.