How do I stop algae in my reservoir or on my medium?

Q: How do I stop algae in my reservoir or on my medium?

Algae in a cannabis reservoir or on the medium means light is reaching wet, nutrient-rich surfaces. Stop it by keeping light out: opaque reservoirs and lids, no gaps around net pots, covered tubing, and a dry or covered top layer on coco and rockwool. Keep the solution at 18–22 °C (64–72 °F). Scrub and disinfect between runs. Algicides and routine peroxide are not the fix.

Short answer

Algae in a reservoir or on the medium means light is reaching wet surfaces that have nutrients on them. Keep the light out and the algae stop. Use opaque reservoirs and lids, close the gaps around net pots, cover tubing, and keep the top of the medium dry or covered. Keep the solution at 18–22 °C (64–72 °F). Established.

Why

Algae need three things: water, nutrients and light. A hydroponic system supplies the first two on purpose, so light is the only one you can take away. Oregon State's DWC guidance is to cover the water surface with opaque material and size the rafts so there are no gaps, because algae thrive wherever wet, light-exposed surfaces have nutrients ^[1].

Warm water adds to the problem. Warm, poorly aerated solution holds less oxygen and favours Pythium root rot ^[2]^[3]. A reservoir that has gone green is often warm as well.

Cleaning between runs removes the biofilm that algae and microbes leave behind. Organic matter neutralises chlorine, so always scrub before you disinfect ^[4].

The numbers

Item	Target
Reservoir and lid	Opaque; no light visible inside with the room lights on
Solution temperature	18–22 °C (64–72 °F); alarm above 24 °C (75 °F) ^[2]
Dissolved oxygen	≥ 6 mg/L ^[3]
Gaps around net pots or rafts	None ^[1]
Cleaning bleach (between runs)	10% household bleach, 10 min, then rinse ^[4]

What to do

With the lights on, look inside the reservoir with the lid closed. Any glow is a leak.
Wrap or paint clear reservoirs, tubing and channels. Use black-and-white film or tape.
Fill gaps around net pots with collars or neoprene inserts.
On coco or rockwool, cover the top with a mat or dry perlite, and stop drippers from pooling.
Cool the reservoir if it runs above 22 °C.
Between runs, scrub, disinfect, rinse and dry. Wear gloves and eye protection, ventilate, and never mix bleach with acids.

Common mistakes

Clear or translucent reservoirs and lines.
Dosing algicide or peroxide instead of fixing the light leak.
Disinfecting without scrubbing first.
Ignoring algae until it clogs the pump.

Full article: Deep water culture (DWC)
Answers: top off with water or nutrients?
Articles: root browning in hydro · root rot and Pythium
Terms: biofilm · shore fly · beneficial-microbe reservoir

Follow-up questions

Is algae harmful to cannabis roots?

Algae is not a root disease in itself, but it is a problem for cannabis. It grows into a biofilm on roots and in lines, stains roots, clogs drippers and pumps, uses up nutrients, breeds shore flies and makes pH harder to hold. Above all it shows that light is reaching the solution, which should never happen.

Should I add hydrogen peroxide to kill algae?

Do not add hydrogen peroxide routinely to kill algae in a cannabis reservoir. It only kills what it touches, the algae return as long as light gets in, and it also kills the beneficial microbes GenetixBay prefers in a living reservoir. Lightproof the system, then clean it properly between runs. Wear gloves and eye protection when handling concentrated peroxide.

Why is my coco turning green on top?

Coco turns green on top when light and nutrient solution meet on a surface that stays wet, usually from frequent fertigation. It is surface algae, harmless to the roots below but a breeding ground for shore flies and fungus gnats. Cover the top with a coco mat or dry perlite, or use slab wraps, and keep the drippers from pooling.

Sources

Nelson M, Langellotto G, Nackley L, 2025. Hydro hints: Deep water culture (EM 9455). Oregon State University Extension Service. extension.oregonstate.edu/catalog/pub/em9455

Colorado State University Extension PlantTalk 2043. Root disease problems and management in hydroponic systems (68–72 °F). planttalk.colostate.edu/topics/miscellaneous/2043-root-disea

University of Kentucky Cooperative Extension. Pythium root rot in hydroponic production (E-706). greenhousehort.ca.uky.edu/sites/greenhousehort.ca.uky.edu/fi

Oregon State University Extension. Hop latent viroid in hemp (EM 9570). extension.oregonstate.edu/catalog/em-9570-hop-latent-viroid-