Aquarium algae: identify them by colour and get rid of them
Updated on August 26, 2026 · ~6 min read
The type of algae tells you the cause: brown is silicate in a young tank, green on the glass is excess light, hair algae is an imbalance between light and nutrients, black brush points to unstable CO₂ and cyanobacteria to poor flow with too many nutrients. You act on the cause, not on the algae.
Algae in an aquarium are not a disease: they are the tank telling you that something is in excess or in short supply. Removing them by hand works for a week; working out which algae it is tells you the cause, and the cause is corrected once.
This is an identification guide. Each type has a typical look, a typical cause and a different remedy — and using the remedy for one on another is the most common way to fight algae for months.
Brown algae, a dust that wipes off (diatoms)
How they look. A brown film on glass, decor and leaves, powdery, which comes away when you run a finger over it. They appear in the first weeks of a new tank.
Cause. Silicate, which new water brings with it. In marine tanks the reference is under 0.1 ppm of silicate, ideally undetectable.
What to do: nothing. They are a normal step in maturation and they end by themselves when the nutrient runs out. Wipe the glass and wait. Anyone fighting them with products is solving a problem that would have disappeared.
If they appear in a mature tank, the cause is external: tap water rich in silicate, or an exhausted silicate resin on the RO unit.
Green film on the glass
How it looks. A uniform green film on the glass, which comes back a few days after cleaning. The plants are doing well.
Cause. Excess light — too many hours, too much intensity, or direct light from a window — with nutrients available.
What to do. Shorten the photoperiod to 6-7 hours (no tank needs more), lower the intensity if the fixture allows it, and move the tank out of direct daylight. Cleaning the glass stays a manual job, and that is fine: the glass is the one place where algae bother nobody.
Green hair algae
How they look. Green filaments, bright, tangling on the plants and coming away by winding them onto a toothbrush.
Cause. An imbalance between light and nutrients, in a planted tank nearly always light (or CO₂) in excess of the available nutrients. They are often the sign of plants that are not growing: if the plants are stalled, light and carbon are left there for the algae.
What to do. Counter-intuitive, but this is the route: do not starve the tank. Check that nitrate is not at zero (in a planted tank 10-25 ppm is fine, the plants need it) and phosphate is not nil (0.5-1.5 ppm in a planted tank). Then reduce light, stabilise CO₂, prune the plants to stimulate growth and increase water changes for a couple of weeks. Manual removal helps because taking out biomass takes out stored nutrients.
Black brush / beard algae (red-black)
How they look. Greyish-black or reddish tufts, short and tough, on the edges of older leaves, on wood and on equipment. They do not come off by scrubbing.
Cause. In most cases unstable or insufficient CO₂ in a planted tank, together with poor flow or a high organic load. They are the algae of a well-lit tank whose CO₂ comes and goes.
What to do. Stabilise CO₂ (with injection, 20-30 ppm; without it, 3-8 ppm and then lower the light), improve flow, remove the worst affected leaves rather than treating them. They are among the most tenacious: old leaves carrying tufts get cut off, not rescued. Objects — decor, tubing — are treated outside the tank.
Cyanobacteria (slimy film, blue-green or red-brown)
How they look. They are not algae: they are photosynthetic bacteria. They form a mucilaginous sheet that peels off in patches, slippery, often with a swampy smell, covering substrate and plants and smothering them.
Cause. Poor water movement and excess nutrients, generally in the tank's dead spots. In marine tanks they often appear in the first months.
What to do. Vacuum the sheet out with a siphon (it comes away in whole patches), increase the flow where it was forming, and reduce nutrients with water changes and less food. A total blackout of 3 days — tank covered, lights off — hits them hard and the plants tolerate it; the fish fast without trouble. ⚠️ Be wary of anyone promising to solve them with antibiotics: they work, but they also hit the filter bacteria, so treat them as a last resort with ammonia monitored over the following days.
Summary table
| Appearance | What it is | Most likely cause | First thing to do |
|---|---|---|---|
| Brown dusty film, new tank | diatoms | silicate from new water | wait |
| Uniform green film on the glass | green algae | too much light | 6-7 hours of light, out of the sun |
| Green filaments on the plants | hair algae | excess light/CO₂, nutrients at zero | do not starve it, reduce light |
| Hard black or red tufts on edges | brush / beard | unstable CO₂, poor flow | stabilise CO₂, cut the leaves |
| Slippery sheet peeling off | cyanobacteria | poor flow + nutrients | siphon, more flow, 3-day blackout |
| Very hard green dots on the glass | green spot algae | high light, low phosphate | scraper, raise phosphate |
The three things that apply to all of them
The photoperiod is the cheapest lever. Six or seven hours are enough for any tank. Lights are not split into two blocks "to let the plants rest": that is a myth, and meanwhile the total time gets longer.
Less food. Everything that goes into the tank and is not eaten becomes nitrate and phosphate. It is the first lever, the most effective and the least practised.
There is no product that solves it. Algaecides work for a few weeks, then the algae come back because the cause is still there — and some harm plants, shrimp and filter bacteria. "Algae-eating" animals (Otocinclus, Siamese algae eaters, snails; in marine tanks hermits and snails) help to contain, not to solve, and each of them is an animal with needs of its own: not a tool.
A tank with no algae at all is not the goal. A thin film on the glass and a few strands on the wood are normal in an ecosystem that works. The signal to watch is not presence, it is fast growth: algae doubling in a week are telling you something changed — the new bulb, the new fertiliser, the water change you skipped.
How Gurglee does it
An algae problem is read on the trend of the nutrients, not on one reading: Gurglee keeps charts of nitrate and phosphate with the band for your type of tank (in a planted tank the lower bound too, because at zero the plants stall and the algae win), and shows the effect of a water change before you make it. If you need an opinion on your specific case there is the tank doctor, which reads that tank's actual readings instead of answering in general terms, and which you can describe your setup to so it takes light, CO₂ and routine into account.
Related guides
Phosphate and algae: the link nobody explains
Why high phosphate feeds algae, where it comes from, which values to hold in freshwater and in a reef, and why driving it to zero does not solve the problem.
High nitrate: how to lower it for real
Why nitrate climbs in an aquarium, which values to hold in freshwater and in a reef, and the methods that actually lower it — from water changes to plants.
Aquarium water change: how much, how often, how
How much water to change and how often, how to prepare the new water and the steps to do it without stressing the fish. With the arithmetic of the effect on nitrate.
Cloudy or milky aquarium water: the 5 causes
Aquarium water gone cloudy, milky, green or yellow: how to tell a bacterial bloom from suspended substrate, and what to do in each case.