Aquarium algae types: how to identify them and what fixes each

Identify the algae first

• black beard algae (BBA, also called black brush algae): dark gray to black tufts on leaf edges, wood and tubing; a red alga despite the color,
• staghorn algae: gray-green branching strands that look like small antlers, on leaf edges and equipment; a red alga of the genus Compsopogon,
• green spot algae (GSA): hard round green dots that will not rub off, on the glass and on older leaves, especially Anubias,
• green dust algae (GDA): a soft green film on the glass that wipes off with a finger and is back within days,
• hair or thread algae: green filaments, long and soft or short and stiff, on plants and decor,
• cyanobacteria (blue-green algae): a slimy blue-green to almost black sheet on substrate and leaves that peels off in one piece and smells,
• diatoms: a dusty brown coating, mostly in the first weeks,
• green water: free-floating single-celled algae that turn the whole tank green.

Diatoms mostly belong to new tanks and are covered in the guide to algae in a new aquarium. Green water has its own section in the guide to cloudy aquarium water. This guide is about algae in a tank that has been running for months.

Why an established tank suddenly gets algae

Plants and algae need the same things: light, CO₂ and nutrients. While those are balanced and plants grow well, algae cannot get a foothold. Shift one of them – a CO₂ cylinder runs out, the photoperiod creeps up, fertilizing stops, more fish go in, the filter slows down or mulm builds up in the substrate – and algae respond faster than plants do.

So the useful question is what changed in the last few weeks. Baseline values for a planted tank:

• CO₂ 20–30 mg/l (ppm), steady through the whole photoperiod, if you inject CO₂,
• nitrate (NO₃) 10–20 mg/l,
• light 6–8 hours a day.

Black beard algae and staghorn

Both are red algae and both tend to appear when CO₂ is low or swings during the day and organic waste builds up: leftover food, dead leaves, mulm in the filter and substrate. BBA often grows at the filter outlet and on equipment but also in dead corners, so sources disagree on whether strong or weak flow is the bigger problem. They do agree that water should move through the whole tank without dead spots.

• stabilize CO₂: the same amount every day for the whole photoperiod, and do not let the cylinder run dry,
• cut badly covered leaves off at the base; they will not clean up,
• siphon mulm off the substrate surface and keep water changes regular; rinse the filter media in water drained from the tank, not under the tap, and not on the same day as a big substrate clean,
• take affected wood or rock out, treat it with 3 % hydrogen peroxide for a few minutes and rinse it thoroughly in dechlorinated water; keep it off moss and plants attached to the wood,
• for spots you cannot remove, liquid carbon, usually glutaraldehyde-based, can be spot-dosed.

Tanks without injected CO₂ get BBA too, and the same logic applies in a different form: keep conditions steady, light closer to 6 hours, change a similar amount of water on a regular schedule rather than occasional large changes, and keep the tank clean.

With liquid carbon, spot doses count toward the daily total on the label for your tank volume, and that total must not be exceeded. Switch the filter off for a few minutes, apply the product with a syringe directly onto the algae, then switch the filter back on. Products differ in strength and at least one manufacturer warns that overdosing it as a spot treatment can damage plants, which is why no figure is given here. Vallisneria and some mosses react badly, and the product can harm shrimp and snails. If fish or shrimp react to the treatment, do a large water change straight away. Treated algae turn pink or white within a few days, and Amano shrimp then clean up what is left; they will not clear a mature, healthy patch on their own.

Dosing hydrogen peroxide into the tank itself is not worth the risk: published doses differ several-fold, and an overdose harms shrimp, fish, sensitive plants and filter bacteria.

Green spot and green dust algae

Green spot algae stick firmly to glass and leaves and hit slow growers like Anubias hardest. They most often go with low phosphate (PO₄) and long or very bright lighting. Test phosphate with a sensitive kit: if it sits at the bottom of the scale or below about 0.5 mg/l, dose a phosphate fertilizer (a rough target is 0.5–2 mg/l, see the fertilizing guide). Very high readings around 3 mg/l or more are a less common cause; then larger water changes help. Then scrape the spots off with a blade scraper and watch whether they return. Nerite snails are the most effective grazers.

Green dust algae sit loosely on the glass. A method that works for many tanks is to leave the film for at least three weeks until it ages and starts to break up, then wipe it all off in one go and do a water change straight away to take the debris out. In aquarists’ experience, wiping it every few days releases spores and the film comes back.

Hair and thread algae

Hair algae grow when there is more light than plants can use with the CO₂ and nutrients they have, or when plants grow slowly for another reason. What helps:

• pull the threads out by hand and trim heavily covered leaves,
• shorten the photoperiod toward 6 hours,
• bring CO₂ and fertilizing back into line so plants grow at full speed,
• Amano shrimp and other dwarf shrimp, which graze the soft filaments but leave short, stiff ones alone.

Algaecides do not remove the cause. Some harm plants, some are labeled as unsafe for shrimp, and copper-based ones kill shrimp and snails, so keep them out of any tank with invertebrates.

Cyanobacteria (blue-green algae)

Cyanobacteria are photosynthetic bacteria, not algae. They thrive where water does not move, in substrate full of organic waste, with overfeeding and sometimes where nitrate has bottomed out.

• siphon out as much of the slime as you can during a water change and clean the substrate surface,
• improve circulation so no corner is stagnant,
• do not let nitrate bottom out: 10–20 mg/l in a planted tank, a measurable 5–10 mg/l in a shrimp tank,
• feed less,
• for a heavy outbreak, siphon out what you can first, then black the tank out for three to four days: cover the sides too, switch CO₂ off, stop fertilizing, add an airstone or raise the filter outlet, feed very little and check on the fish and shrimp. Do a large water change afterwards, because the dying cyanobacteria pollute the water.

A blackout does not fix the cause. If flow, organics and nutrients stay the same, cyanobacteria come back within a few weeks.

How to work through it and what to log

Change one thing at a time and give it at least a week. Photograph the same spot from the same angle under the same light, and note what you changed: photoperiod, CO₂, fertilizer, water changes, new plants or fish. Algae react with a delay, so without notes it is hard to tell what triggered an outbreak and what fixed it.

A photo of the same spot a week later tells you more than memory does.