Aquarium Algae: Types, Causes and Treatment

How do you identify aquarium algae?

You identify algae by colour, texture and where it grows; six types cover almost every case in the hobby references. The list below runs from the green algae most aquariums meet first to the rarer problems.

1. Green spot algae (GSA): small, hard green spots on the glass and on the leaves of slow-growing plants that are difficult to wipe off.
2. Hair or thread algae: green filaments, including staghorn and string algae, that look like wet hair when lifted out of the water. It grows on plant edges and rocks or forms matted clumps.
3. Green water: the whole water column turns pea-soup green from free-floating single-celled algae.
4. Diatoms (brown algae): a dusty, flour-like brown coating on glass, substrate and plants that wipes off easily. It is most common in newly set-up tanks.
5. Black beard or brush algae (BBA): thick, bushy, dark tufts firmly attached to plants, decor and equipment. LiveAquaria compares it to coarse horsehair, and despite its colour it belongs to the red algae.
6. Blue-green algae (cyanobacteria): a slimy sheet over substrate, plants and decor, usually blue-green but also brown, black or red, with an earthy or foul smell that may be noticed before it is visible.

What causes algae in an aquarium?

Algae grows when light, nutrients and CO2 are out of balance, so that plants cannot use everything on offer. Aquarium Co-Op and Tropica both describe plants and algae competing for the same light, nutrients and CO2: with too much light for the nutrients available, or too many nutrients for the light, algae takes the surplus. Over-fertilising the water is one route, because plants cannot take up fertiliser faster than their light allows.

Light

Light is the trigger most within your control. Tropica advises about 6 hours/day of light for the first two to three weeks of a new planted tank, then 8–10 hours/day once plants settle. Aquarium Co-Op allows longer days for established tanks but says to cut back if algae spreads. Direct sunlight and inconsistent hours both feed algae, which is why Co-Op recommends a timer.

CO2 and bright light

The brighter the light, the more CO2 and fertiliser plants need. Aquarium Co-Op says high light of 40–50+ PAR usually needs added CO2, or algae can take over, while low-tech tanks run on low light of about 10–20 PAR. GWAPA links black beard algae to unstable CO2. Our guide to CO2 levels in planted tanks explains how to measure and hold CO2 steady.

Nutrients and waste

Nitrate and phosphate come from fish waste and food as well as fertiliser. The nitrogen cycle turns ammonia from waste into nitrate, which plants use and algae can use too. Aquarium Co-Op calls nitrate below about 40 ppm safe, and Practical Fishkeeping says a reading that far above tap water calls for more water changes, the main way to remove it.

Where the sources disagree

The per-type causes are not settled. For green spot algae, GWAPA blames depleted phosphate and suggests raising it in planted tanks, while LiveAquaria lists high phosphate, excess light or low CO2. For hair algae, GWAPA points to excess iron, LiveAquaria to excess nitrate and phosphate, and Aquarium Co-Op to excess nutrients, too much light or too few nutrients. Treat the trigger column above as a starting point and test before you change dosing; the best aquarium test kits compared cover nitrate and phosphate tests.

How do you get rid of each type of algae?

Remove what you can by hand first, then fix the trigger so it does not come back. The steps below follow Aquarium Co-Op, GWAPA and LiveAquaria for each type.

1. Green spot algae: scrape the glass with a razor blade, or a plastic card on acrylic. Nerite snails eat GSA. In planted tanks GWAPA suggests raising phosphate to a moderate level, while LiveAquaria suggests reducing light.
2. Hair or thread algae: pull it out by hand or with a toothbrush, shorten the photoperiod and rebalance fertiliser. GWAPA links it to iron above about 0.15 ppm.
3. Green water: a UV sterilizer clears it in about two to three days, according to Aquarium Co-Op. The alternative is a blackout: Co-Op recommends at least a week of darkness and GWAPA describes five days. Co-Op notes that large water changes alone do not work, and that light and feeding should be reduced at the same time.
4. Diatoms: wipe them off with a sponge. They usually fade as a new tank matures and silicate and phosphate are used up.
5. Black beard algae: spray affected decor or plants out of the water with 3% hydrogen peroxide, leave about five minutes, rinse and return, and trim affected leaves. GWAPA recommends stable CO2 in planted tanks. Aquarium Co-Op warns that full eradication can take six to eight months.
6. Cyanobacteria: siphon off the slime, vacuum the substrate, clean the filter, improve circulation and stop overfeeding. GWAPA links it to low nitrate in planted tanks. If it returns, Aquarium Co-Op describes erythromycin products such as Maracyn and Fritz Slime Out, each with its own label dose.

Liquid carbon (glutaraldehyde) products are a further option in planted tanks. Seachem’s Flourish Excel label gives a dosing schedule and warns against spot-treating plants, because overdosing can damage them.

Which algae eaters help with each type?

Algae eaters help with soft films and some filament algae, but none of them fixes the imbalance that caused it. The animals below appear in our species data with algae in their recorded diets.

• Nerite snail: grazes algal film on glass and hardscape, and the algae sources name it for green spot algae and diatoms. It needs pH 7.2–8.0 water. The aquarium snail tank mates guide covers which fish leave it alone.
• Amano shrimp: grazes most soft algae and biofilm; Practical Fishkeeping says almost all forms. The algae sources name it for hair algae and black beard algae. It grows to 2 in (5.1 cm) and needs at least 10 US gal (38 L).
• Cherry shrimp: grazes algae, biofilm and detritus, according to Aquarium Co-Op, and the sources list shrimp among diatom eaters. The fish that live safely with cherry shrimp are ranked on their own hub.
• Bristlenose pleco: Practical Fishkeeping notes it rasps algae and biofilm, though PlanetCatfish says it still needs mainly vegetable foods such as algae wafers.
• Molly: the algae sources list mollies among hair algae grazers, and FishBase says the sailfin molly feeds mainly on algae in the wild.

The sources also name Siamese algae eaters, otocinclus and Florida flagfish, which are not yet in our species data. No grazer clears green water, which is suspended in the water column, or cyanobacteria.

Which fish and plants are sensitive to algae treatments?

Plants and the biological filter are more at risk from algae treatments than fish are. The cautions in the sources are listed below.

• Plants and liquid carbon: Seachem says anacharis is sensitive to Flourish Excel and should be dosed every other day, and that high doses can melt Vallisneria leaves.
• Plants and blackouts: Aquarium Co-Op notes that blackouts and antibiotic treatments are hard on live plants.
• The biofilter and antibiotics: UF/IFAS Extension warns that antibiotic baths may harm the biological filter, while Aquarium Co-Op says its erythromycin product does not. Test ammonia and nitrite during and after any erythromycin treatment.

No source in our data addresses the safety of hydrogen peroxide or liquid carbon for shrimp or snails, so remove treated decor and rinse it before it goes back into a tank with invertebrates.

What looks like algae but is something else?

Blue-green algae is the main look-alike: it is not a true alga but a photosynthetic bacterium (cyanobacteria), which is why antibiotic products affect it. Green water is the other common confusion. Its green tint separates an algal bloom from a white bacterial bloom, which is a different problem covered in the guide to cloudy aquarium water. Black beard algae can also mislead, since it belongs to the red algae despite its dark colour. Finally, the marimo ball sold as a plant is itself a green alga, according to Tropica, so it is not a sign of an algae problem.

Will an algae eater solve an algae problem?

No. An algae eater removes some algae, but the light and nutrient imbalance that grew it remains, and some popular bottom fish are not algae eaters at all. Aquarium Co-Op notes that corydoras are not primarily algae eaters, and PlanetCatfish says bristlenose plecos need vegetable foods beyond the algae they find. Practical Fishkeeping notes that algal film is mostly what nerites eat in the wild, so a spotless tank can leave a nerite short of food. Use grazers as maintenance, and fix the photoperiod, CO2 and nutrients as the cure.

When does an algae bloom become dangerous to fish?

A bloom becomes dangerous when it consumes the oxygen fish need. Algae and cyanobacteria consume oxygen at night, and Southern Regional Aquaculture Center pond data shows heavy blooms can stress or suffocate fish. SRAC also lists cyanobacteria among the most common toxin-producing algae in freshwater ponds, though how far that applies to aquariums is not quantified. Algae itself is not a fish disease. If fish gasp at the surface during a bloom or after a treatment, test ammonia, nitrite and oxygen, and ask an aquatic veterinarian before using antibiotics against cyanobacteria.

How do you prevent algae from coming back?

Prevention comes down to steady light, steady nutrients and plenty of plants. Aquarium Co-Op recommends a lighting timer, keeping the tank out of direct sunlight and changing nutrient dosing gradually, waiting two to three weeks between adjustments. LiveAquaria and Co-Op add the routine basics: avoid overfeeding, keep up water changes and gravel cleaning, and keep the water moving.

Planted tanks start with the advantage of plant mass. Aquarium Co-Op and Tropica both advise planting heavily from the start, including fast growers, so plants use up spare nutrients while a new tank settles, and Tropica suggests little or no fertiliser for the first 3–4 weeks. The guide to how to start a planted tank walks through that set-up, and the low-tech planted tank guide covers tanks run without added CO2. The fish disease guides index lists the other tank problems we cover.