Aquarium Algae: Types, Causes and How to Get Rid of Them

Every aquarium has algae. A little green on the glass or rocks is normal and even a sign of a living tank. The trouble starts when algae smother the plants, tint the water or come back every week no matter how much you scrub. It is nearly always a symptom of an imbalance between light, nutrients and plants, which is why algaecides alone rarely fix it. In this guide you will learn to identify each type of algae, what causes it and how to fix the real cause.

Why algae grow

Like plants, algae need light, nutrients (nitrate, phosphate, iron and others) and CO2. The difference is that they adapt much faster. When something gets out of balance, they are the first to take advantage. The most common causes are:

• Too much light: more than 8-10 hours a day, a fixture that is too strong for the plants you keep, or direct sunlight on the tank.
• Excess nutrients: leftover food, infrequent water changes, a dirty filter or uncontrolled fertilizer dosing.
• Few plants, or plants that are not growing: without competition, algae win.
• Imbalance in planted tanks: lots of light and fertilizer but little CO2, or strong light with a nutrient deficiency.
• A young tank: in the first weeks after cycling it is normal to see diatoms and some green algae while the system settles.

Types of algae: cause and fix

Identifying the algae helps a lot, because each type points to a different cause. This table covers the most common ones in freshwater:

Algae	How to recognize it	Usual cause	How to fight it
Green spot algae	Hard little green dots on the glass and on slow-growing leaves like anubias	Strong light, low phosphate, slow plants	Scrape the glass, cut light hours, fertilize in balance
Hair algae	Long green threads that tangle around plants and decor	Too much light and nutrients, unstable CO2	Remove by hand with a toothbrush, less light, more fast-growing plants
Black beard / black brush algae	Very tough black or gray tufts on leaf edges, driftwood and tubing	Low or fluctuating CO2, built-up organic waste, poor flow	Trim affected leaves, stabilize CO2, clean the filter and improve circulation
Diatoms (brown algae)	Dusty brown film that wipes off with a finger	New tank, silicates, low light	Patience: they usually disappear on their own within weeks; wipe and let otocinclus and snails work
Cyanobacteria (blue-green algae)	Slimy blue-green or reddish sheets with a strong smell	Dead spots with no flow, very low nitrate, organic waste	Siphon it out, improve flow, frequent water changes, a 3-day blackout
Green water	Water turns green like pea soup	Free-floating microalgae fed by too much light plus ammonia or nutrients	Cut light, a blackout of several days, UV sterilizer, water changes

Note that cyanobacteria are not algae at all but photosynthetic bacteria. That is why algae-eating fish and snails leave them alone, and why they respond to different fixes, such as better water circulation.

Light and nutrients: the balance that matters

Light is the fastest and cheapest thing to adjust. Keep a photoperiod of 6-8 hours a day on a timer, starting at 6 in new tanks or tanks with algae. If you already have an outbreak, dropping to 5-6 hours for two or three weeks usually slows the algae without hurting the plants. Dimmable LED aquarium lights help a lot: often the problem is not the hours but the intensity. The details are in how many hours of light an aquarium needs.

As for nutrients, the goal is not to strip the water to zero but to give plants what they need with nothing left over. Some guidelines:

• Nitrate between 10 and 25 ppm in a planted tank; near 0 encourages cyanobacteria and green spot, and above 40 ppm feeds algae.
• Phosphate in moderation: in well-growing planted tanks, low but detectable levels prevent green spot algae.
• Fertilizer: dose liquid fertilizer according to how many plants you have and how strong the light is. More light needs more fertilizer and, above a certain intensity, CO2.
• Food: only what the fish eat in 2-3 minutes. Leftover food is the main source of phosphate.

Plants are your best algae control. Fast growers such as Hygrophila difformis, limnophila or floating plants like salvinia take up nutrients quickly and provide shade. Slow plants such as anubias and ferns look great but compete very little.

Algae eaters: realistic expectations

Algae-eating animals are an excellent helper, but they do not fix the cause. If the tank has too much light or too many nutrients, algae will grow faster than they can eat it. These are the most useful in community tanks:

Animal	Eats well	Does not eat	Keep in mind
Amano shrimp	Fine hair algae, leftovers and biofilm	Mature black beard, green spot	Works best in groups (about 1 per 3-5 gallons); needs tankmates that will not eat it
Otocinclus	Diatoms and soft algae film on leaves and glass	Hair algae, black beard	A schooling fish (6 or more) and delicate; only for mature tanks with algae or supplemental food
Nerite snail	Diatoms, soft green spot, film on glass and rocks	Long hair algae, black beard	Does not breed in freshwater, but leaves white eggs on decor

Avoid buying fish just "to clean": some, like the common pleco, grow far bigger than most home aquariums can hold. We cover this in fish that clean the aquarium.

Chemical algaecides: the last resort

Algaecides can knock back a severe outbreak, but as long as the cause remains, the algae will return. They also have side effects: dying algae decompose and can cause oxygen drops, and some products harm sensitive plants, shrimp or the biological filter.

For green water, a UV sterilizer works better than any chemical. For black beard algae, liquid carbon products spot-dosed onto the affected areas work in many cases, always following the manufacturer's directions.

Common mistakes

• Leaving the lights on 10-12 hours "so the fish can see."
• Placing the tank next to a window with direct sun.
• Using an algaecide without changing anything else: the outbreak returns within weeks.
• Suddenly stopping fertilizer in a planted tank: plants stall and algae take over.
• Buying a pleco for a 15-gallon tank expecting it to clean everything.
• Deep-cleaning the filter and substrate at the same time, which unsettles the bacteria and spikes nutrients.
• Changing many things at once: make one adjustment, wait two weeks and observe.

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