# Green slime or algae on the medium, roots or in the reservoir: likely causes and fixes

> Green slime or algae on the medium, roots or in the reservoir on cannabis is most often Algae from light reaching wet surfaces or the reservoir. How to tell them apart, the fix for each and what recovery looks like.

Also called: algae, green stuff on soil, green reservoir, slime in res, green film on rockwool.

Run the checker with this sign: https://genetixbay.com/knowledge/diagnose/?at=roots&s=green-slime

## Likely causes, most likely first

### Algae from light reaching wet surfaces or the reservoir (Likely)

- Kind: Root zone; urgency: Low urgency
- Fix: Block all light from reservoirs, tubing and the medium surface (opaque lids, covers on rockwool). Clean surfaces between runs. Algae competes for oxygen and feeds gnats and shore flies.
- Recovery: No new green growth within 1-2 weeks of light-proofing.
- Confirm: Look at the roots: Hydro: lift the net pot and look at the root mass in good white light. Containers: tip the plant out with a hand over the medium and look at the edges and bottom of the root ball. Healthy roots are white to cream with fine fuzzy tips. Brown roots whose outer layer slides off between the fingers leave a bare thread: rot. Firm, evenly tan roots with white tips in coloured solution are staining.
- Article: [How do I stop algae in my reservoir or on my medium?](https://genetixbay.com/knowledge/answers/algae-in-reservoir.md)

## Note

Guidance, not a guarantee. The checker weighs common causes against what you tell it; confirm with the tests before you treat, change one thing at a time and watch the new growth.