Green spot algae (and green dust algae): how to remove it and stop it coming back

Green spot algae is the hard green dots that stick to your glass and to slow plants like Anubias. It is harmless to fish. Scrape it off the glass with a blade, and test your phosphate, because phosphate that is too low (or, less often, too high) is the usual cause.

Green dust algae is different: a soft green haze that wipes off easily but comes back within days. The trick with green dust algae is the opposite of what you'd expect: leave it alone for a few weeks, then clean it all off in one go.

How to identify it

	Green spot algae	Green dust algae
Looks like	Small, round, hard green dots, like polka dots, that can merge into a crust	An even, dusty green haze over the glass
Wipe test	Does not come off with a sponge or your finger	Comes off easily with a finger or a swish of water
Where	Glass, equipment, rocks and slow-growing leaves (Anubias, Java fern, Bucephalandra)	Mostly glass, sometimes decor
Comes back	Slowly, over weeks	Within hours to days of wiping

Green spot algae is usually thought to be Coleochaete, a green alga that grows as tiny flat plates. Green dust algae is made of microscopic algae that haven't been firmly identified; they swim actively as spores and resettle quickly.

Not the same: brown algae (diatoms) is a soft brown film common in new tanks. Hair algae grows as threads.

When it's a problem

Normal: a few green spots on the glass every week or two in a well-lit tank. A quick scrape is part of routine care.
Early: spots appearing every few days, or starting on plant leaves.
Serious: older Anubias and fern leaves crusted over. The plants grow so slowly that algae blocks their light, and the leaves never fully clean up.
Emergency: never, on its own. These algae don't harm fish.

What to do right now

For green spot algae

Scrape the glass with a razor-blade scraper or a dedicated algae blade. Use a plastic blade on acrylic tanks, because metal will scratch them. Keep the blade flat and watch the silicone in the corners.
Remove badly covered old leaves from slow plants. Cut them at the base.
Test phosphate if you have a kit, plus nitrate.
Shorten the lights to about 6 to 7 hours a day while you sort out the cause.

For green dust algae

Stop wiping it. Each wipe releases spores into the water, and they settle again on the clean glass.
Clean only the front glass if you must, and accept that it will come back.
Mark the date on a calendar and plan a deep clean about three weeks later.

What to do next

Green spot algae: balance phosphate and light

Planted-tank references link green spot algae to phosphate, but they disagree on the direction. One view says phosphate that is too low in a brightly lit tank is the main cause, and the fix is to add a little phosphate fertiliser. Another says either extreme can trigger it: too low, or high readings around 3 mg/L or more. Commonly quoted healthy ranges for planted tanks run from about 0.1 to 2 mg/L. So:

If phosphate reads zero or nearly zero and you have strong light and plants, a fertiliser that contains phosphate may help.
If phosphate is high, do larger water changes, feed less, and check whether your tap water carries phosphate.
If you can't test, reduce light hours first, and keep water changes regular.

Low CO2 and other nutrient shortfalls in planted tanks can also play a part. Once you've changed something, scrape the glass clean and watch whether new spots form. Within two to three weeks of stable conditions, new spots should slow down.

Nerite snails

Nerite snails (and related horned and zebra nerites) are the most useful clean-up crew for green spot algae. They graze it off glass and leaves slowly but steadily. They don't breed in fresh water, although females may lay small white eggs on decor that won't hatch. Shrimp and most algae-eating fish make little impact on hard spots.

Cleaning plant leaves

You can't scrape Anubias leaves hard without damaging them. Gently rub with your fingers, let nerites work on them, or trim the worst leaves. Some keepers dip removable plants for a few minutes in very dilute hydrogen peroxide and rinse well. Go gently; it can damage delicate plants.

Green dust algae: wait, then wipe

Green dust algae runs through a life cycle. Experienced aquascapers recommend leaving it completely alone for about two to four weeks (sources range from 14 days to four weeks). Over that time the film thickens, goes patchy and starts peeling off. Snails and suckermouth catfish may graze it. Then:

Lower the water level as far as is safe for your fish.
Wipe all the glass thoroughly, including the back and sides, and clean the filter intake.
Do a large water change, around 50%, to remove free-floating spores.

If it returns quickly, check for recent big changes: a new light, a heavy trim, or a sudden jump in fertiliser.

Products that help

Razor-blade algae scraper (plastic blade for acrylic): the fastest way to remove green spot algae from glass.
Phosphate test kit: tells you whether phosphate is low, normal or high.
Phosphate-containing plant fertiliser: for planted tanks where phosphate tests near zero.
Phosphate-removing filter media: only if testing shows phosphate is high and water changes alone don't bring it down.
Lighting timer: keeps light hours short and steady.
Nerite snails: slow, steady grazers of spot algae.

How to prevent it

Keep the light on 6 to 8 hours a day.
Don't put slow plants like Anubias in the brightest spot; they do well in shade.
Test phosphate now and then if you fertilise a planted tank.
Change 25 to 50% of the water weekly.
Scrape the glass weekly before spots build up.
Avoid sudden big changes in light, plants or fertiliser.
Keep a few nerite snails.

Why it happened

Green spot algae grows on surfaces that sit still in good light: glass, rocks and leaves that grow too slowly to outrun it. A phosphate imbalance or long, bright lighting tips things its way. Green dust algae tends to follow sudden changes such as a big trim, a new light or a nutrient spike.

When to get help

Ask a planted-tank club or experienced aquascaper if spots keep coming back fast after a month of stable light and corrected phosphate. Take your test results and a photo.

In the game

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