Identify Your Algae: Green, Brown, Hair, and Black Beard

Quick answer

Algae is not a plague to be scrubbed away — it is a message. Each type points to a specific imbalance between light, nutrients and CO2. Brown diatoms mean a young, immature tank. Green spot and green dust usually mean strong light. Hair and thread algae point to excess nutrients or unstable CO2. Black beard algae signals fluctuating CO2 and weak flow. Identify the type first, correct the underlying imbalance, and only then clean up what is left.

Algae is the number one frustration for aquarium keepers, but each type tells you something different about your tank.

Why algae appears

Algae is not a disease. It is the visible result of an imbalance between the three things every photosynthetic organism needs: light, carbon dioxide, and nutrients (nitrogen, phosphorus, potassium and a handful of trace elements). Spores of many algae species are always present in the water and on new plants — you cannot keep them out. What decides whether they bloom is opportunity.

In a healthy planted tank, the plants act as a living sponge, soaking up light and nutrients so fast that little is left over for algae. Trouble starts when that balance tips: the light is stronger or longer than the plants can use, CO2 swings up and down, or nutrients pile up faster than the plants take them in. Algae, which is far less fussy than higher plants, simply exploits the surplus. This is why the single most reliable long-term algae control is not a chemical but a tank full of thriving, fast-growing plants. Read the type of algae and you know which lever — light, CO2 or nutrients — has slipped out of proportion.

Read the tank: an algae ID table

Before you treat anything, match what you see to the cause. This is the fastest way to stop guessing.

Brown algae (diatoms)

A soft brown dust coating glass, plants and decor is diatom algae, and it is the classic sign of a young, still-maturing tank. Diatoms build their cell walls from silica, so they feed on the silicates that leach from fresh substrate, new sand and even some tap water. In the first weeks a new tank also has swinging parameters and no established competition, which is exactly what diatoms like.

The good news is that diatoms are self-limiting. As the tank matures — biofilm settles, plants root, silicate leaching slows — they usually fade on their own within two to eight weeks. They wipe off effortlessly, and a small cleanup crew clears them fast: otocinclus catfish and nerite snails graze diatoms enthusiastically. If a mature tank suddenly browns over, suspect a new silicate source (a fresh bag of sand, a switch in tap water) or dropping light.

Each algae type has a distinct look that hints at its cause.

Green algae

Green algae is the largest family and comes in several forms, each with its own tell. Green spot algae (GSA) makes hard green dots on the glass and on slow-growing leaves such as anubias; it points to strong light combined with low phosphate, and counterintuitively a small phosphate increase often slows it. Green dust algae (GDA) films the whole pane in a fine green haze; the patient fix is to leave it undisturbed for one to three weeks until it matures, sloughs off and is easy to wipe. Green water is a free-floating bloom that turns the tank into pea soup, usually triggered by too much light, an ammonia pulse or excess nutrients; it clears with a three-day blackout or a UV clarifier, but not by water changes alone since the spores multiply too fast.

Hair and thread algae

Soft green strands that grow in tufts or long threads on plants and hardscape are hair or thread algae. In a planted tank they are usually driven by excess nutrients, light stronger than the plants can use, or unstable CO2 that leaves plants unable to compete. Manual removal is genuinely effective here: twirl the strands onto a toothbrush like spaghetti, then tighten your light and nutrient control so it does not simply grow back. Amano shrimp graze hair algae with real appetite and are the classic biological control. Note that stiff, dark-green branching strands that feel wiry — Cladophora — are a stubborn cousin that shrimp mostly ignore and that needs patient manual removal.

A well-chosen cleanup crew grazes many algae types before they spread.

Black beard algae (BBA)

Dark, blackish-purple tufts that feel tough and cling stubbornly to leaf edges, wood and equipment are black beard algae — despite the colour it is a red algae (Rhodophyta). It thrives where CO2 fluctuates and flow is weak, which is why it so often appears right at a filter outflow, on the oldest leaves, and on hardscape in dead spots. The cure is rarely scrubbing: stabilise your CO2 so it stays steady through the photoperiod, improve circulation so every corner gets flow, and spot-treat affected hardscape out of the tank. A liquid-carbon or diluted-peroxide dab (applied with the pump off, following the product label) turns BBA red then grey as it dies. Siamese algae eaters are among the very few fish that will actually eat it.

The main schools of algae control

Ask ten experienced hobbyists how to beat algae and you will hear several genuinely different philosophies. None is "wrong" — they suit different tanks, budgets and temperaments.

There are regional flavours too. The Nature Aquarium school, with its emphasis on lush low-nutrient scapes and a large shrimp cleanup crew, grew out of Takashi Amano's work in Japan and is dominant across East Asia. Estimative Index was popularised in the United States and remains the default among high-tech planted-tank keepers there and in the UK. Low-tech Walstad methods have a strong following among European and North American hobbyists who want a quiet, low-cost tank. The practical lesson is the same everywhere: fix the imbalance with light, CO2 and plant mass, and use grazers or manual work to mop up — chemicals are a last resort, not a first reflex.