Algae: what kind you have and how to fix it

Identification guide for common aquarium algae with actual causes and real fixes, not product recommendations.

Algae isn't a disease. It's a symptom. Every type has a cause, and the cause is almost always an imbalance between light, nutrients, and CO2. The fix is correcting the imbalance, not buying an algaecide.

Green spot algae

Looks like: Hard green dots on glass, slow-growing plant leaves (anubias, java fern), and hardscape. Doesn't form filaments. Feels gritty when you scrape it.

Cause: Low phosphate relative to light. GSA appears when phosphate bottoms out while light and other nutrients stay adequate. Common in tanks that don't dose phosphate separately, or that run phosphate-removing filter media.

Fix: Dose phosphate. Monopotassium phosphate (KH2PO4) is the usual dry powder. A common planted-tank target is one to two ppm of phosphate in the water column, replenished two or three times a week. In a tank running the EI (Estimative Index) method, GSA rarely appears because phosphate stays in excess. Scrape existing spots off the glass with a razor blade; on plant leaves, cut away the affected leaves, since GSA doesn't lift off soft tissue easily.

Black beard algae (BBA)

Looks like: Dark grey to black tufts, 1-3 cm long, branching filaments. Grows on leaf edges, filter outlets, driftwood, and any surface with flow. Feels fuzzy.

Cause: Fluctuating CO2. BBA's signature trigger is inconsistent CO2, not low CO2 by itself. A tank that holds CO2 steady through the day rarely gets it, while a tank whose CO2 climbs and crashes across the photoperiod gets it reliably. Usual culprits: CO2 running out before lights-off, poor diffuser placement leaving dead zones, or DIY CO2 with erratic output.

Fix: Stabilize CO2. Check that the diffuser puts out a consistent stream from lights-on to lights-off. Make sure the CO2-enriched water reaches every part of the tank (a circulation pump helps in large tanks). For existing BBA, spot-treat with a syringe of liquid carbon (Seachem Excel or generic glutaraldehyde) squirted straight onto the tufts with the filter off. The tufts turn red-pink within a few days and die. Siamese algae eaters (Crossocheilus oblongus, not the lookalikes) are one of the few fish that actually eat BBA, though they won't clear a heavy outbreak on their own.

Staghorn algae

Looks like: Grey-green branching filaments resembling deer antlers. Stiffer than hair algae, more branched than BBA. Grows on leaf edges and filter components.

Cause: Similar to BBA. Low or unstable CO2, often paired with weak flow in parts of the tank. Tends to appear in the same conditions as BBA but favors slightly different surfaces.

Fix: Same approach as BBA. Stabilise CO2, improve circulation, and dab liquid carbon onto the worst growth. Amano shrimp eat young staghorn if you catch the infestation early.

Green hair algae

Looks like: Long (2-10 cm) soft green filaments, often tangled in clumps on plants, hardscape, and substrate. Easy to pull out by hand or wind around a toothbrush.

Cause: Excess light relative to nutrients or CO2. Very common in new tanks with strong lights and not enough plant mass to compete for nutrients. Also shows up in established tanks during nutrient imbalances, particularly when iron is dosed without matching macronutrient levels.

Fix: Cut the photoperiod by 1-2 hours. Dose macronutrients (N, P, K) so plants aren't starved while the algae has surplus light to exploit. Pull out as much as you can during water changes. Amano shrimp are the best biological control for hair algae; a group of ten in a tank of around 75 litres can clear a moderate outbreak in a couple of weeks. Nerite snails graze it too, but slowly.

Green water

Looks like: The water itself turns green, from a faint tint to full pea-soup opacity. Not attached to surfaces; it's free-floating single-celled algae (Chlorella, Euglena, and similar).

Cause: A combination of excess light, surplus dissolved nutrients (often ammonia or ammonium from a new or disrupted nitrogen cycle), and no competition from plants. Very common in new tanks and after a major disturbance (filter cleaning, substrate stirring, a medication that killed the bacterial colony).

Fix: A UV sterilizer is the fast route. An inline UV unit on the canister return clears the free-floating algae in a few days. Without UV, large daily water changes for a week dilute the algae faster than it reproduces. A blackout (no light for several days, tank fully covered) also works but stresses plants. Daphnia eat green-water algae and can clear a tank quickly, though most fish eat the daphnia before the daphnia eat the algae.

Brown diatoms

Looks like: Dusty brown coating on glass, substrate, plants, and decor. Wipes off easily. Comes back within a day or two.

Cause: Silicate and low light. Diatoms build their cell walls (frustules) out of dissolved silicates drawn from tap water, new substrate, or new rock, and they outcompete green algae in dim conditions. Extremely common in new tanks during the first couple of months, then they fade on their own as the silicate runs down and the tank matures.

Fix: Wait. In most tanks, diatoms are a passing new-tank phase that resolves without intervention. Otocinclus eat them efficiently and are the standard biological control. Nerite snails graze them too. Nudging the light up a little (if the tank is very dim) tips the balance toward green algae and plants, which outcompete the diatoms. Don't bother scrubbing repeatedly; they return until the silicate source is spent.

Blue-green algae (cyanobacteria)

Looks like: A slimy sheet, usually dark green, blue-green, or purple-black, coating substrate, plants, and hardscape in a wet-looking film. Peels off in sheets. Smells bad (earthy, musty, rotten). Despite the name it is not a true alga; it's a photosynthetic bacterium.

Cause: Low nitrate combined with heavy organic waste. Cyanobacteria thrive where nitrogen is scarce and true plants and algae struggle to compete. Tanks with nitrate sitting near zero, sluggish circulation, and detritus rotting in the substrate are the usual breeding ground.

Fix: Raise nitrate and fix the flow. Potassium nitrate (KNO3) is the standard source. Bring nitrate up into the low tens of ppm. Step up water flow, especially along the substrate where the sheets form, and siphon the cyano off during water changes. In stubborn cases an erythromycin-based product sold for cyanobacteria can clear it, but treat that as a last resort. Erythromycin is a macrolide antibiotic: it spares aquarium plants but also knocks back the nitrifying bacteria in your filter, so follow the product label exactly and watch ammonia and nitrite closely during and after the course.

The general pattern

Most algae problems share the same root: too much light for the available CO2 and nutrients. Plants can't use the surplus light, but algae can. The sequence of fixes that resolves most situations:

1. Cut the photoperiod to 6-7 hours.
2. Dose a complete fertilizer (macro and micro) so no nutrient is rate-limiting.
3. If you inject CO2, confirm it's stable and adequate. Pushing CO2 too high suffocates fish, so raise it gradually and keep watching the livestock.
4. If you don't inject CO2, keep the light moderate and stock plants from the low-demand list.
5. Add biological controls (amano shrimp, otocinclus, nerites) suited to the algae type.

The lighting calculator and nutrient dosing calculator can help balance the light and nutrient inputs.