Why Is My Aquarium Growing Algae? Causes and What to Change

The short answer

Algae needs light and nutrients, so an outbreak usually means it is getting more of them than your plants and upkeep use: lights on too long or too bright, direct sunlight, or nitrate and phosphate from food, fish waste or tap water. New tanks often go through a brown diatom phase, and some algae in a mature tank is normal.

Aqueon, an aquarium equipment maker, puts it plainly: “Algae are aquatic plants in their most basic form”, and like all plants they need “water, light, minerals and nutrients to grow”. Laura Muha, writing in TFH, a fishkeeping magazine, adds that the spores cannot be kept out (“algae are everywhere”), so what you can change is the light and the fuel.

What feeds a sudden algae outbreak

Lights on too long. Aqueon gives “a maximum of 6 to 8 hours of light” for unplanted aquariums and 10 to 12 hours for planted ones. Aquarium Co-Op, an aquarium retailer, starts a newly planted tank at “only 6–8 hours a day”, raises it “up to 8–12 hours a day” as plants grow, and says “If algae starts getting out of control, then decrease the duration again.”
Light that is too strong or aging. Aquarium Co-Op, an aquarium retailer, warns that a planted-tank LED at 100% brightness “may be too strong and cause algae to flourish”, depending on the plants and the tank’s height. Aqueon adds that algae prosper under aging fluorescent tubes where many plants do not.
Sunlight. Aqueon advises against a spot that gets direct sun “even if it’s only for a few hours a day”. In Muha’s freshwater tank, placed where the winter sun did not reach, by summer “one end of the tank got a couple of hours of sunlight every day”, and she began to notice more algae.
Nitrate and phosphate. Aqueon names these as the main algae nutrients, coming from fish food and waste “but can also be present in tap water”. API, a maker of test kits and water treatments, notes that water treatment facilities “may add phosphate to tap water to prevent pipe corrosion”.
Few or no live plants. Aqueon finds algae more common where nutrients are high, “especially if there are no live plants”.
A new tank. Brown diatoms are, in Aqueon’s words, “often seen in newly set up aquariums”.

Which algae is it?

What you see hints at what feeds it:

What you see	What it is	What feeds it
Hard green dots on glass and slow-growing leaves	Green spot algae	Too much light or a phosphate imbalance (Aquarium Co-Op)
Strands like wet hair on plants, wood or rock	Hair, string, thread or staghorn algae	Phosphate build-up (Aqueon); excess iron, too much light, or too few nutrients for the light hours (Aquarium Co-Op)
Soft brown dust that wipes off	Diatoms	New tanks (Aqueon); phosphates and silicates (Aquarium Co-Op)
Dark green or blue-green slime, often a musty smell	Cyanobacteria, “actually not algae” (Aqueon)	Often seen where organic waste stagnates; the cause is not settled (Aquarium Co-Op)
Black or gray bushy clumps	Black beard algae	Many possible causes; not many things eat it (Aquarium Co-Op)
Pea-soup green water	Free-floating algae	Nitrate and phosphate plus too much light (Aqueon)

The common types and their fixes

Green spot algae. Aquarium Co-Op says the spots are very difficult to clean off and suggests a “gift card (for acrylic tanks)” among its tools; on glass, use an algae scraper made for aquariums.

Hair, string and staghorn algae. For planted tanks, Aquarium Co-Op suggests “decreasing your lighting period, increasing fertilization, or decreasing iron”. Aqueon says longer strands can be removed “by twirling them around a toothbrush”, and leaves covered in them should be trimmed out.

Brown diatoms. In a newly set up tank, Aqueon says they “will usually die off on their own after the tank cycles”; Aquarium Co-Op notes they rub off easily with an algae scrubber sponge. Our guide to brown stuff in an aquarium covers diatoms and other brown deposits.

Blue-green slime. Aquarium Co-Op sees this cyanobacteria “wherever organic waste has a chance to stagnate”: slow current, a lit corner closed off by hardscape, or unvacuumed gravel. Its fixes: siphon or scrub off what you can, change water more often, clean the filter regularly, feed less if you overfeed, and improve flow (a stronger filter, a powerhead, moved decorations). Algae eaters “typically will not eat the stuff”, and Aqueon notes siphoned slime often returns quickly.

Green water, which Aqueon says water changes clear only for a while, has its own guide: green, cloudy aquarium water.

What to change, step by step

Tip

Change one thing at a time. Muha held back because “if I fixed everything all at once, I’d never know which factors were contributing most”.

Test the tank water and the tap water you use for changes

When algae becomes excessive, Aqueon advises testing both for nitrate and phosphate; its targets are nitrate “below 10 ppm” and phosphate “below 0.5 ppm”. This guide is about freshwater tanks; Aqueon’s page does not say which tank type those numbers are for, and saltwater and reef tanks have their own targets. API’s Freshwater Master Test Kit has no phosphate test; API makes a separate Phosphate Test Kit. In a heavily planted tank, read the planted-tank section before cutting nutrients. If nitrate reads high, our guide to high nitrate in an aquarium covers its causes and fixes.

Check ammonia while you test

In an established, cycled tank, Muha writes, any ammonia means “you have a bigger problem than algae to worry about”, such as overstocking, overfeeding or a dead fish somewhere in the tank. Deal with that first; our guide to high ammonia in an aquarium covers the usual causes.

Put the light on a timer

Aqueon recommends a timer “to provide a consistent photoperiod”: 6 to 8 hours for an unplanted tank; for a planted tank, follow the Co-Op steps under “Lights on too long” above. Aquarium Co-Op says “do not leave the light on all night, not even in blue moonlight mode”. In a planted tank, do not cut the light so far that the plants suffer, which Aquarium Co-Op likens to killing a lawn over a few weeds.

Turn down the brightness and block the sun

For a dimmable planted-tank LED, Aquarium Co-Op suggests starting “around 20–40% brightness” and raising it only if no algae grows; without a dimmer, raise the light above the tank or tape over some LEDs. For a tank near a window, Aqueon suggests blinds or curtains closed during the day, or a background covered with thin insulation board or cardboard. If your light uses fluorescent tubes, Aqueon says to change them “every 10 to 12 months”.

Feed sparingly and do not overstock

Aqueon’s way to keep nitrate and phosphate low starts with “avoid having too many fish, feed sparingly”.

Keep up water changes and filter care

Aqueon advises “regular water changes using nitrate and phosphate free water” and cleaning the filter regularly. In an established tank, Buce Plant, an aquarium plant retailer, suggests replacing 20 to 30% of the water weekly. A new tank is different: Aqueon’s page on cloudy water in a new tank says “Regular partial water changes are the #1 thing aquarists should do to be successful, EXCEPT during New Tank Syndrome” (its name for the cloudy-water bloom of the first weeks), and that doing nothing is best “as long as ammonia and nitrite levels are not on the rise”; test the water before you skip a change. For nutrient-rich tap water, Aqueon suggests reverse osmosis or deionized water with a product such as Aqueon Water Renewal. In a new tank, leave a brand-new filter alone: Aqueon’s new-tank advice is that cleaning it or replacing its media “does nothing good, and potentially eliminates the good bacteria that are trying to get established”.

Add more live plants, including fast growers

Aqueon says plants work best “when the aquarium is heavily planted”, naming “hornwort, wisteria and teardrop Rotala” among the fast growers. They need bright light, and Aqueon says they “should not be placed in aquariums containing herbivorous fish or those that dig excessively”.

Remove what is already there, gradually

Scrape the walls (an aquarium algae scraper on glass, a gift card on acrylic), twirl strands onto a toothbrush, siphon slime and trim covered leaves. Muha removed hers slowly, since algae was acting as filtration, “to give the beneficial bacteria in the biofilter time to pick up the slack”.

Consider algae eaters as a helper, not a cure

Aquarium Co-Op matches them to the type: nerite snails for green spot (its algae-eater guide calls them “one of the few animals capable of removing and eating it”); otocinclus, snails and shrimp for diatoms; Siamese algae eaters, Amano shrimp, mollies and Florida flagfish for hair algae. Check each animal’s needs first: Aquarium Co-Op says plecostomus “often get very large as adults”, Amano shrimp eat algae “only if you don’t feed them too much”, and flagfish can sometimes damage delicate plant leaves; Aqueon warns some plecos eat broadleaf plants like Amazon swords and Anubias.

Planted tanks: the balance works differently

Aquarium Co-Op says plants and algae use the same light, nutrients and carbon dioxide, and calls algae “an imbalance of nutrients and lighting”. So “sometimes the better course of action is to increase one or both of them”. Green Aqua, an aquascaping shop, goes further: “excess nitrates and phosphates are not the main culprits, unless at very high levels which harm plant growth”, and it says green spot algae blooms when “Phosphate and/or CO2 levels are low”. So in a planted tank, weigh Aqueon’s nutrient targets against these views, make one change at a time and, as Aquarium Co-Op advises for low-tech tanks, wait two weeks before the next.

Algaecides: a last resort, label first

Aqueon says chemical algaecides “should never be the first choice”, as they do not address the cause, and “should always be used as a last resort”. Its instruction: “Always follow product directions and remove carbon and other chemical media from your filter before using algae control chemicals.” For green water, it warns that sudden algae die-offs “can spike ammonia and nitrite levels”.

Warning

Read the label of the exact product first. API’s AlgaeFix, sold for freshwater aquariums, says: “Do not overdose.” “Caution: do not use with freshwater crustaceans, including shrimp, crabs, and lobsters.” “Siphon or scrape any dead algae from aquarium.” “It is a violation of federal law to use this product in a manner inconsistent with its labeling.” If you keep shrimp, such as the Amano shrimp in step 9, that label rules it out.

Is some algae fine?

Yes, within limits. Aqueon says “some algae growth in a mature aquarium is normal” and calls algae “nature’s way of purifying water and removing pollutants”. It becomes a problem when it covers plant leaves (Buce Plant notes this “may block light”), keeps spreading, or turns out to be cyanobacteria. If your fish look unwell, test the water and contact an aquatic veterinarian; algae control works on the water and equipment, not on the fish.

Ready to buy?

Our roundup of freshwater aquarium test kits compares kits for the nitrate test in step 1, and our Water Care & Testing section gathers the test kits and conditioners. To clear algae that is already there, see our guide to getting rid of algae in a fish tank.

Frequently Asked Questions

Q. Will algae go away on its own?

Aqueon says brown diatoms in a new tank usually die off once it has cycled. For other types, the sources here fix the light, nutrients or flow rather than wait.

Q. Does algae in a fish tank kill fish?

The sources differ on cyanobacteria: Aquarium Co-Op says it “does not usually harm fish”, while Aqueon says that, left unchecked, it can “suffocate live plants and even cause harm to fish”. If fish look unwell, test the water and call an aquatic veterinarian.

Q. What kills algae immediately?

Scraping, siphoning and trimming remove what you can see the same day. Aqueon warns that without fixing the cause, “sooner or later the problem will return”, and it keeps algaecides as a last resort.

Q. How can I get rid of algae naturally?

Aquarium Co-Op’s approach is to bring light and nutrients as close to balance as you can, then “use an algae-eating crew to fill in the rest of the gap”.

Sources (12)

Aquarium Tales Team

Written and reviewed by the Aquarium Tales Team. See the About page for how we work.