Algae is almost never a sign that your tank is dirty. It is a sign of imbalance — too much light, too many nutrients, too little CO2 relative to demand, or biology not yet established. Identifying which kind of algae you have tells you which imbalance to address. Scrubbing the glass weekly without fixing the underlying cause is treating the symptom while feeding the disease.
The diagnostic step nobody does
Algae blooms are not random. They appear because something in the tank’s chemistry and biology favors them over plants (in a planted tank) or microbial competitors (in any tank). Five inputs drive almost every algae problem:
- Light duration and intensity
- Nitrate and phosphate levels
- CO2 availability (in planted tanks)
- Established biological filtration
- Flow patterns (dead spots breed algae)
Match the algae type to the cause before deciding how to fix it.
Common algae types
Green dust algae and green spot algae
Green coating on the glass, sometimes spotty, sometimes diffuse. Normal in any tank. Wipe with a magnetic scraper or razor blade weekly. Not a problem unless the rate of accumulation is extreme.
Cause: some light is reaching surfaces. Healthy tanks all have it.
Brown algae (diatoms)
Brown dusty coating on plants, glass, substrate. Appears in new tanks, almost always within the first 2-3 months.
Cause: silicate in tap water or substrate, immature biological filter. Resolves on its own as the tank matures — within 2-6 weeks for most setups. Otocinclus catfish eat diatoms aggressively. Snails (nerite, mystery) help.
Green water
Cloudy green water — looks like pea soup. Free-floating microscopic algae.
Cause: excess light, excess nutrients, often after a tank disruption. The fastest fix: a UV sterilizer. Within 3-5 days, water clears. Without UV: blackout for 5-7 days (cover the tank completely, no light), heavy water changes, and reduced feeding.
Black beard algae (BBA)
Tufts of dark green to black fuzz growing on slow-growing leaves, driftwood, and equipment. Difficult to remove physically. Among the most-feared algae for planted-tank keepers.
Cause: low or fluctuating CO2 in planted tanks; sometimes excess organic waste in non-planted tanks.
Fix:
- Stabilize CO2 (planted tanks)
- Spot-treat with liquid carbon (Seachem Excel, Easy Carbo) — turn off filters, syringe directly onto algae, leave 5 minutes
- Siamese algae eaters (true SAE, Crossocheilus oblongus) eat BBA — one of the few fish that does
- Remove badly-infected hardscape and bleach-dip (10% bleach, 5 minutes, rinse thoroughly, soak in dechlorinator)
Cyanobacteria (blue-green algae / BGA)
Despite the name, not algae — it is bacteria. Slimy mat-like growth, dark green to bluish, peels off in sheets. Distinctive musty smell. Spreads fast and smothers plants.
Causes: low nitrate, low flow, excess organic waste, low oxygen.
Fix: erythromycin treatment (Maracyn) is the only reliable solution. 4-day course. Combined with improved water flow and a thorough substrate vacuum, this resolves most cases. Manual removal alone never works long-term — the bacterial colonies regrow.
Hair algae and string algae
Long stringy filaments, light to medium green, attached to plants, decor, glass.
Cause: excess light, nutrient imbalance (often too little CO2 relative to nutrients), insufficient plant mass to outcompete.
Fix:
- Remove manually — twist around a toothbrush or chopstick
- Reduce light duration to 6 hours per day for 2-3 weeks
- Amano shrimp eat hair algae aggressively
- Add more fast-growing plants to outcompete
Staghorn algae
Coarse, gray-green branching tufts that look like deer antlers. Less common but persistent.
Cause: low CO2 in planted tanks, often coupled with high organic waste.
Fix: spot-treat with liquid carbon, stabilize CO2, improve flow. Slower to resolve than other algae.
Algae eaters that actually help
- Otocinclus catfish — diatoms and soft green algae. Schooling, must keep 4+.
- Amano shrimp — hair algae, biofilm. Best invertebrate algae eater.
- Nerite snails — every kind of algae on glass and hardscape. Cannot breed in freshwater (no population explosion).
- Siamese algae eaters (true SAE) — one of few fish that eat black beard algae.
- Bristlenose plecos — green algae on glass and decor. Stay small (under 5″).
Algae eaters that do not help (despite marketing)
- Common pleco — grows to 18″, eats little once over 6″
- Chinese algae eater — becomes aggressive as it matures, stops eating algae
- Mystery snails — eat detritus and some algae, but won’t solve a real algae problem
Prevention
- Light timer set to 6-8 hours per day. Longer hours feed algae more than they help plants.
- Don’t place tanks near windows. Direct sun is the #1 driver of algae in many home setups.
- Heavy plant load. Plants compete with algae for nutrients. A densely planted tank has fewer algae problems than a sparsely planted one.
- Regular water changes. 30-50% weekly removes the dissolved organics that feed many algae types.
- Don’t overfeed. Excess food becomes excess nutrients becomes excess algae.
- Vacuum substrate. Detritus under the surface feeds algae blooms.
Bottom line
Identify the algae before treating. Brown algae goes away on its own. Green dust is normal. Cyanobacteria needs erythromycin. Hair algae responds to shorter light periods and Amano shrimp. Black beard algae needs CO2 stability and SAEs. Green water responds to UV or blackout. Every algae fix starts with reducing the input that favors it — usually light duration, then excess nutrients, then flow.