Why Algae Grows: Understanding the Root Cause
Algae are photosynthetic organisms — like plants, they need light, CO2, and nutrients to grow. They are also opportunists that exploit imbalances between these three factors. When light and nutrients are abundant but plants are absent or sparse, algae fills the ecological vacuum. When lighting runs too long, nutrients accumulate faster than plants consume them, algae proliferates. When CO2 is insufficient for plant growth, plants become outcompeted by algae that are more efficient at low CO2 levels.
The solution framework for all algae problems follows the same logic: (1) identify the specific algae type — each type signals a different imbalance; (2) address the root cause of the imbalance; (3) physically remove as much algae as possible; (4) introduce biological controls (algae-eating animals); (5) prevent recurrence through correct lighting, nutrients, and CO2 management.
Some algae is inevitable and normal in any established aquarium — a thin green coating on the glass is healthy and shows the tank's biological processes are working. The problem is out-of-control algae that covers plants, dominates the aquascape, and signals systemic imbalance.
Green Spot Algae (GSA) — Hard Green Dots on Glass and Leaves
Green spot algae appears as small, hard, dark green circles firmly attached to the glass, slow-growing plant leaves, and decorations. It is a sign of low phosphate levels relative to other nutrients — the tank has sufficient light but insufficient phosphate for plants to use (causing localized phosphate deficiency). Add phosphate supplement (Seachem Flourish Phosphorus, 1-2 ml per 100 liters weekly) and observe whether GSA growth slows over 2-3 weeks.
Nerite snails are the best biological control for GSA — they are one of the few aquarium animals with a mouth hard enough to scrape GSA off glass. Two or three nerite snails in a 100-liter tank will keep the glass clear of GSA without requiring manual scraping. They do not breed in freshwater, so population control is not a concern.
Scrape existing GSA with a razor blade algae scraper (safe for glass tanks) or plastic scraper (for acrylic). Remove the scraped material with a siphon rather than leaving it to sink and decompose.
Black Beard Algae (BBA) — The Aquascaper's Nightmare
Black beard algae (Audouinella, sometimes called red algae despite being dark gray-black) appears as dense tufts or coatings on plant leaves, filter intakes, driftwood, and decorations. It is virtually impossible to remove mechanically — pulling it off plants damages the plant itself. BBA is a direct indicator of CO2 fluctuation or deficiency — it thrives precisely where CO2 levels dip below the threshold for plant photosynthesis.
Primary treatment: increase and stabilize CO2 injection. BBA cannot compete with healthy, fast-growing plants in a CO2-supplemented environment. Secondary treatment: spot-dose liquid carbon (Excel or EasyCarbo) directly onto BBA using a pipette or syringe with the filter turned off — apply, wait 5 minutes, then turn flow back on. Within 3-5 days, treated BBA turns red and dies. Repeat weekly until clear.
SAEs (Siamese Algae Eaters, Crossocheilus oblongus) are the only fish that reliably eat BBA once it is dying or dead from liquid carbon treatment. True SAEs are difficult to find — confirm species carefully, as many look-alikes (like Chinese algae eaters) are often sold under the same name and do not eat BBA.
Blue-Green Algae (Cyanobacteria) — The Slimy Mat
Blue-green algae (BGA) is not actually algae — it is cyanobacteria, a photosynthetic bacteria that forms thick, smelly blue-green or purple-brown mats over the substrate, plants, and decorations. It typically signals very low nitrate and/or low flow areas where organic matter accumulates. BGA can grow in near-darkness using nutrients from decomposing organic matter.
Remove as much BGA physically as possible with a siphon. Increase flow in affected areas to prevent dead zones. Perform large water changes (50%) to dilute nutrients. If BGA persists, treat with Erythromycin (an antibiotic specifically effective against cyanobacteria) — it eliminates BGA within 3-4 days but may temporarily stress the biological filter. Follow erythromycin treatment with a large water change and probiotic bacterial supplement.
Root cause: organic waste accumulation, low nitrate, stagnant areas, or overly long photoperiod. Clean the substrate thoroughly (gravel vac every water change), increase filter flow rate, shorten the photoperiod to 6-7 hours, and perform larger weekly water changes.
Green Water (Suspended Algae), Hair Algae, and Diatoms
Green water (pea soup algae) makes the tank water bright green and opaque — it is a bloom of microscopic suspended algae, usually triggered by new tank setup, overfeeding, direct sunlight hitting the tank, or a sudden spike in nutrients from an organic source. A UV sterilizer eliminates green water within 48-72 hours by killing the suspended organisms. Diatom filters also clear green water rapidly. Reduce feeding, eliminate direct sunlight, and perform a 50% water change to address the underlying nutrient excess.
Hair algae (filamentous green algae) forms long, tangled green threads — it looks like wet hair growing on plants and decor. It is caused by excess nutrients (especially iron and CO2 fluctuation). Manual removal (twist hair algae onto an old toothbrush), liquid carbon spot treatment, and introducing Amano shrimp (which specifically eat hair algae) in large numbers (1 per 5 liters) controls it effectively.
Diatoms (brown dust) appear as a brown-orange coating on all surfaces in new tanks. They are normal during the first 4-8 weeks of a new setup and typically disappear on their own as the tank matures and live plants establish. Otocinclus catfish (also called Otocinclus algae eaters or "otos") eat diatoms voraciously and are excellent biological control in any tank where conditions suit them (clean, soft, slightly acidic water with plants and no aggressive fish).
Algae-Eating Fish and Invertebrates
A comprehensive algae control team for a planted community tank: Otocinclus catfish (diatoms, soft algae on glass and leaves), Nerite snails (GSA, diatoms — cannot over-breed in freshwater), Amano shrimp (hair algae, detritus, soft algae — most effective algae eater per gram of body mass in the hobby), Bristlenose pleco (glass algae, biofilm — stays small at 12-15 cm unlike common plecos that reach 50 cm).
Avoid common plecos for planted tanks — they grow enormous, uproot plants, and produce more waste than they clean. Chinese algae eaters grow aggressive and territorial with age. True Siamese algae eaters (SAE) are effective at BBA and hair algae but require larger tanks (60+ liters) and careful feeding to remain interested in algae once aquarium food is available.
In Cambodia, Otocinclus and Nerite snails are available at most aquarium specialty shops in Phnom Penh. Amano shrimp and SAE availability varies — check with specialty shrimp and planted tank shops.