Aquarium Algae: How to Identify It and Get Rid of It
Algae is not a cleanliness problem — it is a symptom. Every type tells you something specific about light, nutrients or circulation, and the fix is different for each.
In This Guide
Why Algae Appears
Algae grows when light and nutrients are available and nothing else is using them. That is why a new tank, a tank by a window, and an overfed tank all get algae for what looks like the same reason but isn't.
Too much light
More than 8 hours a day, or direct sun. The single most common cause.
Excess nutrients
Overfeeding and infrequent water changes leave nitrate and phosphate for algae to use.
No competition
Growing plants outcompete algae for the same nutrients. A bare tank has no competitor.
New tanks get algae. That's normal.
Almost every tank goes through a brown diatom phase in its first two months while the biology settles. It passes on its own.
Identify What You Have
| Looks like | Type | What it's telling you |
|---|---|---|
| Brown dust on glass and plants, wipes off easily | Diatoms | A new tank finding its balance, or silicates in the water. Usually resolves in weeks. |
| Hard green dots on the glass, needs a blade | Green spot algae | Light is high and phosphate is low. Common in planted tanks. |
| Soft green threads on plants and decor | Hair / thread algae | Too much light for the nutrients available. Often follows a lighting upgrade. |
| Dark tufts on leaf edges and hardscape, very tough | Black beard algae | Unstable CO2 and poor circulation. The hardest type to remove. |
| Green water, tank looks like pea soup | Free-floating algae | An ammonia spike plus strong light. Water changes alone make it worse. |
| Blue-green slimy sheets, smells earthy | Cyanobacteria | Not algae at all — a bacterium. Low flow and organic build-up in the substrate. |
Cyanobacteria is the one to act on quickly
It spreads across the substrate in days, suffocates plants, and no fish or shrimp will eat it. Improve flow, siphon it out physically, and blackout the tank for three days.
Fix the Cause First
Adding algae eaters to a tank whose lighting is wrong just gives you hungry fish and the same algae. Work through these in order.
Cut the photoperiod to 6-8 hours
Use a timer. Consistency matters more than duration — a fixed 7 hours beats a variable 5 to 10.
Move the tank out of direct sunlight
Even an hour of direct sun a day will outgrow anything you do to the light fixture.
Feed less
Feed what the fish finish in about a minute, once a day. Uneaten food is pure algae fertiliser.
Change water on schedule
25% weekly removes the nitrate and phosphate algae depends on.
Add fast-growing plants
Floating plants and stem plants consume the same nutrients and shade the tank at the same time. The most effective long-term fix there is.
Only then, add a cleanup crew
With the cause addressed, algae eaters keep the remaining growth in check instead of fighting a losing battle.
Choosing a Cleanup Crew
Algae eaters are specialists. Pick the one that eats the type you actually have, and check it fits your tank.
| Species | Minimum tank | Adult size | Temperature | pH | Group size |
|---|---|---|---|---|---|
| Otocinclus | 10 gal / 38 L | 2 inches / 5.1 cm | 72-79°F / 22-26°C | pH 6.0-7.5 | 6+ |
| Amano Shrimp | 10 gal / 38 L | 2 inches / 5.1 cm | 70-80°F / 21-27°C | pH 6.5-7.5 | 6+ |
| Nerite Snail | 5 gal / 19 L | 1 inch / 2.5 cm | 72-78°F / 22-26°C | pH 7.0-8.5 | — |
| Siamese Algae Eater | 30 gal / 114 L | 6 inches / 15 cm | 75-79°F / 24-26°C | pH 6.5-8.0 | 3+ |
| Bristlenose Pleco | 20 gal / 76 L | 5 inches / 13 cm | 73-81°F / 23-27°C | pH 6.5-7.5 | — |
| Cherry Shrimp | 5 gal / 19 L | 1 inch / 2.5 cm | 65-80°F / 18-27°C | pH 6.5-8.0 | 10+ |
| Rubber Lip Pleco | 20 gal / 76 L | 4 inches / 10 cm | 71-78°F / 22-26°C | pH 6.5-8.0 | — |
- •Otocinclus — soft green film and diatoms on leaves. Needs an established tank with existing algae, and a group of at least six.
- •Amano Shrimp — hair algae, the best in the trade at it. Effective from about five individuals.
- •Nerite Snail — green spot algae on glass, which almost nothing else will touch. Lays harmless white eggs that won't hatch in freshwater.
- •Siamese Algae Eater — one of the very few animals that eats black beard algae, but it reaches 15 cm and needs a 30 gallon tank.
- •Bristlenose Pleco — general green film on glass and wood. Stays at 12 cm, unlike the common pleco it is often confused with.
The two to avoid
The Common Pleco is sold as an algae eater and reaches 45 cm, at which point it stops eating algae and produces enormous amounts of waste. Chinese algae eaters become aggressive and start rasping at other fish.
What Doesn't Work
- •Turning the lights off entirely for weeks. It kills your plants first, which removes the only real competitor algae has. A three-day blackout for cyanobacteria is the exception.
- •Algaecides as a first resort. They kill algae faster than the tank can process the dead matter, causing an ammonia spike, and many are toxic to shrimp and snails.
- •Scrubbing everything weekly. Physical removal helps, but if the cause is untouched the algae returns in days and you have only stressed the tank.
- •Replacing all the water at once. A large sudden change shocks fish and can restart the cycle without touching the underlying nutrient load.
The realistic goal
A completely algae-free aquarium is not the target and is not achievable. A tank with a little algae on the back glass and none on the plants is a healthy, normal aquarium.
Frequently Asked Questions
How do I get rid of algae in my aquarium?
Reduce lighting to 6-8 hours a day on a timer, move the tank out of direct sunlight, feed less, change 25% of the water weekly, and add fast-growing plants. Add algae eaters such as otocinclus, amano shrimp or nerite snails only after the cause is addressed.
What is the best algae eater for a freshwater tank?
It depends on the algae. Nerite snails handle green spot algae on glass, amano shrimp handle hair algae, otocinclus handle soft green film on plants, and siamese algae eaters are one of the few species that eat black beard algae.
Why does my new aquarium have brown algae?
Brown dust on the glass and plants is diatoms, and almost every new tank gets it in the first two months while the biology settles. It usually disappears on its own; otocinclus and nerite snails will clear it faster.
Is blue-green algae dangerous?
Blue-green algae is cyanobacteria, not algae. It spreads quickly, smothers plants and has a distinctive earthy smell, and no fish or invertebrate will eat it. Increase water flow, siphon it out physically and black the tank out for three days.
Find the right cleanup crew
Check any algae eater against the fish you already keep before adding it.