Green slime on the reservoir wall, a slippery film in the NFT channel, a green crust on the rockwool slab. It looks cosmetic, which is exactly why people live with it for months. In practice algae change three system parameters at once: oxygen at night, pH during the day, and organic load all the time. The good news is that algae need exactly three things to grow, and one of them is easy to take away in hydroponics.
Quick glossary: Algae - photosynthetic microorganisms that in hydroponics grow anywhere light reaches nutrient solution. DO - dissolved oxygen, measured in mg/L. Fouling - a layer of living organisms on equipment surfaces; algae are the visible part, bacterial biofilm the invisible one.
Three Conditions for Growth, and Only One You Can Remove
Algae need water, dissolved nutrients and light. The first two cannot be removed from a hydroponic system: they are the system. That leaves light, and it turns out to be the only lever that works reliably.
Hence the conclusion worth starting any algae problem with: this is a light-sealing job rather than a disinfection job. A product removes what has already grown and changes nothing about the cause. Two weeks later the film is back in the same place, because light reaches it exactly as it did before.
What Algae Actually Cost You
They take oxygen at night. During the light period algae release oxygen and the DO reading looks fine. Once the lights go off they switch to respiration and consume oxygen alongside the roots. The low point falls at the end of the dark period, which is when nobody is measuring.
The thresholds from the dedicated article are worth repeating: below 4 mg/L the root is already suppressed, below 2 mg/L conditions turn anaerobic.
They raise pH during the day. Photosynthesis pulls carbon dioxide out of the solution, carbonic acid goes with it, and pH climbs. At night the process reverses. The article on alkalinity covers three causes of rising pH, and algae add a fourth with a signature of its own: a daily swing rather than a steady drift. If pH climbs by evening and comes back down by morning, look for green on the walls rather than at acid demand.
They create organic load. Algae do not live long, and the dead mass becomes food for bacteria. That directly lowers ORP and drags along everything a falling redox potential drags with it.
They feed flies. Shore fly larvae eat algae specifically, and the appearance of that fly means the system holds plenty of green film and permanently wet surfaces. The same conditions feed the fungus gnat, which does not stop at algae.
They block narrow passages. Drippers, emitters, nozzles, the film on the channel floor. Sensors deserve a separate mention: growth on an EC or pH electrode produces a slow drift in the reading, and the system starts correcting the solution against a wrong number.
Where Light Gets Into the System
Light actually gets into the system through translucent or white reservoir walls, gaps in the reservoir lid, empty NFT channel holes, exposed substrate surface between plants, clear hoses, and standing water on trays and the floor under the lamps, and each spot needs its own fix.
| Location | Why light reaches it | What to do |
|---|---|---|
| Reservoir | white or translucent plastic passes light straight through | opaque container or wrap; white outside, black inside |
| Reservoir lid | gaps around hoses and cables | seal the cutouts rather than leaving them "almost closed" |
| NFT channel | empty net pot holes | plugs in every unused site |
| Substrate surface | slab exposed between plants | cover sheet or film with cut openings |
| Hoses | clear PVC, convenient for watching flow | opaque hose or shade over that run |
| Trays and floor | standing puddles under the lamps | slope and active removal rather than waiting for evaporation |
Microgreens and seedlings are the separate case. There the substrate is exposed to light by definition, there is nothing to cover it with, and the only workable approach is surface drying between waterings plus bottom watering instead of top watering. A permanently wet mat under a lamp will always grow algae.
Dealing With What Has Already Grown
Mechanically. Brush the film off, rinse, drain. This step is not optional: any oxidizer is consumed by organic matter, and the more mass you leave behind, the smaller the share of the product that reaches its target.
Hydrogen peroxide. Working doses for hydroponics are given in the ORP article and sit in the 0.003-0.01% range. Peroxide removes suspended cells and lifts redox potential, while doing little against a dense film on a wall.
UV-C. Treats only the water that passes through the chamber. On reservoir walls, slab surfaces and channels it does nothing at all, which is why UV-C disappoints so predictably as an algae measure.
Copper. It works, and that is exactly why it is dangerous: copper is a micronutrient in the recipe, and an overdose produces phytotoxicity before it clears the algae. For a food crop on a short cycle this is not the tool to start with.
Three Mistakes That Cost the Most
Fighting with products while the light stays on. The common sequence: flush, clean, green again in two weeks, raise the dose. As long as light reaches the solution, any treatment is a consumable rather than a fix.
Treating the film as cosmetic because the plants look fine. The plants will look fine until DO crashes before dawn, or until pH swings far enough for automatic correction to start chasing itself. The film is an indicator of conditions rather than a stain on plastic.
Cleaning the reservoir without touching the sensors. EC and pH electrodes foul first and report a shifted number silently. The tank is clean, the solution is correct according to the instruments, and the plant receives something else entirely. Cleaning and re-calibration belong in the same job as washing the system.
How to Know Algae Are Under Control
Reservoir walls, channel and substrate surface show no green film on a weekly visual check. pH draws no daily swing: evening and morning readings differ within measurement error. DO is measured at the end of the dark period rather than during the day and holds above 5 mg/L. There are no shore flies over the trays. Drippers deliver the same flow along the whole line, and electrodes calibrate without an offset you have to compensate for every time.
If you want to go deeper: Biofilms in Hydroponics: Why the System "Grows Over" and How to Stop It explains the invisible half of the fouling, the half a brush does not lift, and Fungus Gnats: Life Cycle, Diagnosis, and Controlling Larvae in the Substrate shows what arrives when green film sits around long enough.