Algae: what kind you have and how to fix it

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 are adequate. Common in tanks that don't dose phosphate separately, or that use phosphate-removing filter media.

Fix: Dose phosphate. KH2PO4 (monopotassium phosphate) is the standard dry salt. Target 1-2 ppm PO4 in the water column, dosed 2-3 times per week. In a tank with the EI (Estimative Index) dosing method running, GSA rarely appears because phosphate is always in excess. Scrape existing spots off the glass with a razor blade; on plant leaves, remove the affected leaves since GSA doesn't come 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 per se. A tank with stable CO2 at 15 ppm rarely gets BBA. A tank where CO2 swings between 10 and 25 ppm throughout the day gets it reliably. Common causes: CO2 running out before lights-off, poor diffuser placement creating dead zones, or DIY CO2 with inconsistent output.

Fix: Stabilize CO2. Check that the diffuser produces consistent output from lights-on to lights-off. Ensure the CO2-enriched water reaches all parts 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) applied directly to the tufts with the filter off. The tufts turn red-pink within a few days and die. Siamese algae eaters (Crossocheilus oblongus, not the imposters) are one of the few fish that eat BBA, but they won't control a heavy outbreak alone.

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 combined with low 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. Stabilize CO2, improve circulation, spot-treat with liquid carbon. Amano shrimp eat young staghorn if the infestation is caught 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 appears in established tanks during nutrient imbalances, particularly when iron is dosed without matching macronutrient levels.

Fix: Reduce the photoperiod by 1-2 hours. Dose macronutrients (N, P, K) to ensure plants aren't nutrient-starved while algae has excess light energy to exploit. Manually remove as much as possible during water changes. Amano shrimp are the best biological control for hair algae; a group of 10 amanos in a 75L tank can clear a moderate hair algae problem in two weeks. Nerite snails graze it but slowly.

Green water

Looks like: The water itself turns green, from 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, excess dissolved nutrients (usually ammonia or ammonium from a new or disrupted nitrogen cycle), and no competition from plants. Very common in new tanks and after a major tank disturbance (filter cleaning, substrate disturbance, medication that killed the bacterial colony).

Fix: A UV sterilizer is the fast fix. An inline UV unit on the canister filter return kills the free-floating algae in 3-5 days. Without UV: massive water changes (80-90%) daily for a week dilute the algae faster than it can reproduce. Blackout (no light for 4-5 days, tank fully covered) also works but stresses plants. Daphnia eat green water algae and can clear a tank in days, but 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 use dissolved silicates (from tap water, new substrate, or new rock) to build their cell walls. They outcompete green algae in low-light conditions. Extremely common in new tanks during the first 2-8 weeks, then disappear on their own as silicates are consumed and the tank matures.

Fix: Wait. In most tanks, diatoms are a temporary new-tank phase that resolves without intervention. Otocinclus eat diatoms efficiently and are the standard biological control. Nerite snails also graze them. Increasing light slightly (if the tank is very dimly lit) shifts the competitive advantage toward green algae and plants, which outcompete the diatoms. Do not scrub and scrub; they'll come back until the silicate source is exhausted.

Blue-green algae (cyanobacteria)

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

Cause: Low nitrate combined with high organic waste. Cyanobacteria thrive in low-nitrogen conditions where true plants and algae can't compete. Tanks with nitrate consistently below 5 ppm, poor circulation, and decomposing detritus in the substrate are the typical breeding ground.

Fix: Dose nitrate. KNO3 (potassium nitrate) to bring nitrate up to 10-20 ppm. Increase water flow, especially along the substrate where cyano sheets form. Manually remove as much as possible during water changes (siphon it off the substrate). In severe cases, a 3-day blackout combined with erythromycin treatment (erythromycin is an antibiotic that kills bacteria, including cyanobacteria, without harming plants. It may reduce nitrifying bacteria populations temporarily, so monitor ammonia and nitrite during and after treatment) resolves it. Erythromycin is available as API E.M. Erythromycin in the US. Dose per the package instructions for a 3-day 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 light, algae can. The sequence of fixes that resolves most situations:

1. Reduce photoperiod to 6-7 hours.
2. Dose a complete fertilizer (macro and micro) to ensure no nutrient is rate-limiting.
3. If running CO2, verify it's stable and adequate (20-30 ppm).
4. If not running CO2, keep the light moderate and stock plants from the low-demand list.
5. Add biological controls (amano shrimp, otocinclus, nerites) appropriate to the algae type.

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