Planted Tank Algae Troubleshooting

How do you work out what is causing algae?

Algae is a symptom of imbalance rather than a disease to attack. Before changing anything, write down what you see and what has changed recently: a new light setting, more fish, a missed water change, a new fertilizer, or a CO2 adjustment. Recent changes are often the most useful clue.

Then work through it in order:

1. Identify the algae. Use the table and descriptions below.
2. Check the basics. Test ammonia, nitrite, and nitrate, and confirm the tank is cycled. Our cycling and water testing guide covers this.
3. Change one thing at a time. Adjust light first, since it is the easiest to reverse.
4. Give it time. Plants and algae both respond over days to weeks.

Algae quick reference

The triggers below are factors commonly associated with each type in the hobby, not certain causes. Tanks differ, and more than one factor is often involved.

Common types in more detail

Why does a new tank get brown algae?

Brown diatoms are very common in the first weeks of a new tank. They are often linked to silicates and to the ammonia that appears while the filter is still maturing. They usually fade on their own as the tank stabilizes and plants grow in. Wipe them off and keep up regular water changes.

Green spot algae

Green spot algae forms tough, round spots, mostly on glass and on slow-growing leaves such as Anubias. It sticks firmly and needs a scraper on glass. It is often associated with low phosphate or low CO2. A little green spot on old leaves is common even in healthy tanks.

Green dust algae

Green dust forms a soft film on the glass that seems to return within a day of wiping. A common approach among keepers is to stop wiping for a while, let it build into a thicker layer, then scrape it off in one session and siphon out the loose algae during a water change.

Hair and thread algae

Long green strands that tangle around stems and hardscape are commonly associated with excess light, and sometimes with nutrient imbalance or unstable CO2. Twirl strands out with a toothbrush or tweezers, reduce the photoperiod or intensity, and make sure plants are well fed so they can outcompete it.

What causes black beard algae?

Black beard algae forms dark, brush-like tufts, often on leaf edges, wood, and around filter outlets. It is widely considered one of the harder types to remove. In CO2 tanks it is often associated with fluctuating CO2 levels; in all tanks, poor flow and organic buildup are common suspects. Trim heavily affected leaves, clean equipment, and focus on stable conditions.

Green water

Green water is free-floating algae suspended in the water column. Water changes alone often do not clear it because it regrows quickly. Reduce lighting, check for ammonia, avoid overfeeding, and be patient. Some keepers use a blackout or a UV clarifier; follow the equipment maker's instructions if you do.

Cyanobacteria

Often called blue-green algae, cyanobacteria are actually bacteria that photosynthesize. They form slimy sheets with a distinctive smell, often in areas of low flow or around organic debris. Siphon out as much as you can, improve circulation, clean the substrate surface and filter, and reduce feeding.

How do you get rid of algae in a planted tank?

• Reduce the photoperiod or intensity. This is the gentlest first step. See our light intensity and duration guide.
• Stabilize CO2. In injected tanks, aim for consistent levels each day rather than large swings. In low-tech tanks, avoid frequent large changes in maintenance routines that shift dissolved carbon.
• Provide adequate nutrients. Plants that are short of nutrients grow slowly and leave room for algae. Our fertilizer basics guide covers balanced dosing.
• Keep up maintenance. Regular water changes, filter cleaning, and removing dead leaves reduce the organic load.
• Remove algae by hand. Scrape glass, trim affected leaves, and siphon out loose algae during water changes.
• Add fast-growing plants. Stem plants and floating plants can compete with algae for light and nutrients.

Will algae eaters solve the problem?

Snails, shrimp, and some fish graze on certain algae and can help keep it in check. A few cautions:

• Compatibility matters. Check that any algae eater suits your tank size, temperature, water chemistry, and existing fish. Some larger algae eaters outgrow small tanks or become territorial.
• They help with maintenance, not root causes. Grazers rarely fix an imbalance on their own.
• Do not add animals to an uncycled tank. Make sure ammonia and nitrite are reading zero before adding livestock.
• Each type has preferences. Many grazers ignore black beard algae and green spot algae.

Common mistakes

• Reacting to every new patch of algae with a new product or a big change.
• Increasing light to make plants outgrow algae, which often feeds the algae instead.
• Cutting fertilizer sharply, which can starve plants and make algae worse.
• Adding algae eaters to a new tank before it has cycled.
• Changing several variables at once, so you cannot tell what worked.
• Expecting zero algae; a small amount is normal in a living tank.

Next steps

Start with the light: shorten the photoperiod with a reliable schedule, using our timers and lighting schedules guide. Confirm water quality with the test kits hub or a kit from our test kit picks, including phosphate tests, and review dosing in the fertilizer basics guide. If you are planning a new setup, the setup finder can help you choose a balanced combination of light, CO2, and substrate from the start.