Troubleshooting: Algae and slime

What is this algae in my reef tank, and how do I beat it?

Green strands on the rock, green spots or film on the glass, shiny green bubbles, or brown dust. A new tank goes through several of these in turn, which keepers call the ugly phase.

Last checked 21 October 2026. How we check

What should I check first?

1. Which kind is it? Use the list of causes below: each one has its own fix.
2. How old is the tank? Under about six months, some algae is expected.
3. Test nitrate and phosphate, and note the trend over the last few weeks, not one reading.
4. Check your RO/DI output with a TDS meter: it should read 0.
5. Count the hours the main lights are on.

What is most likely causing it?

Most likely first. The order is our reading of how often the sources link each cause to this symptom.

1. Green hair algae: more nutrients than the grazers and corals use, or phosphate leaching from rock. Long green strands on rock, often after a period of heavy feeding, or on new dry rock that leaches phosphate (keeper reports, typical, check).
2. Diatoms: the brown dust of a new tank, fed by silicate. Brown dusty film on sand and glass in the first weeks. See the diatoms page.
3. Green film algae on glass. Green spots or haze on the glass that come back within days of cleaning. Normal in any lit tank; only the speed it returns tells you anything.
4. Bubble algae (often called valonia). Shiny green bubbles from a few millimetres to marble size, stuck to rock. They usually arrive on rock or frag plugs.
5. Not algae at all: cyanobacteria or dinoflagellates. Red or purple sheets that peel off (cyanobacteria), or brown snotty strands with bubbles that vanish at night (dinoflagellates).

What should I do?

1. Remove what you can by hand or with a toothbrush during a water change, and siphon it out so it does not spread.
2. Find the source: cut feeding a little, rinse frozen food, check the RO/DI, and test the rock if it is new dry rock.
3. Add grazers in steps as the algae appears, not all at once. Practical Fishkeeping gives about one Turbo snail per 100 litres, one Trochus per 50 litres and one scarlet reef hermit per 25 litres. See the Trochus snail, Turbo snail and scarlet reef hermit.
4. For bubble algae, Practical Fishkeeping notes an emerald crab may eat it, though some never do. Remove bubbles whole: a burst bubble can spread it.
5. Keep nitrate and phosphate measurable but moderate. Taking them to zero swaps algae for other problems, including dinoflagellates and pale corals.

What not to do

• Buying a fish to do the job unless the tank can feed it for life: a lawnmower blenny needs a mature tank with plenty of algae.
• Stripping nutrients to zero with strong chemical media in a young tank.

When should I get help?

Something dying right now? Go to the emergency playbooks.

Where do sources disagree?