Algae blooms can appear overnight and are common in both new and established tanks. The good news: they’re almost always fixable once you identify the cause.
This guide walks you through the four most common algae types, what causes blooms, and a step-by-step treatment plan to clear your tank and keep algae from coming back.
What Type of Algae Is Taking Over Your Tank?
Start by identifying which type you’re dealing with — each has a different cause and treatment approach.
Green Water (Algae Bloom)
Your water turns cloudy and green, like pea soup. This is caused by billions of suspended single-cell algae floating in the water column. It is most common in tanks receiving too much light combined with excess nutrients. Green water is distinct from a bacterial bloom, which produces a white or milky cloudiness.
Brown Diatoms
A golden-brown dust on glass, substrate, and decorations. Very common in new tanks during the first 2-4 weeks. Harmless and usually clears on its own once the tank matures.
Black Beard / Brush Algae
Dark, tufted growth on plants, hardscape, and equipment. Hard to eradicate. Appears when CO2 levels fluctuate in planted tanks.
Blue-Green Algae (Cyanobacteria)
Slimy blue-green sheets with a foul odor. Not true algae but bacteria. Linked to poor water flow and high organics — requires different treatment than true algae.
Top 5 Causes of Algae Blooms
Most algae outbreaks trace back to one of these five factors.
Too Much Light
Direct sunlight, lights on 12+ hours/day, or outdated bulbs with a shifted spectrum. Algae responds to light more aggressively than most plants. Fix: reduce photoperiod to 8 hours max and use a timer.
Excess Nutrients (Nitrate and Phosphate)
Overfeeding produces uneaten food that breaks down into nitrate — a direct food source for algae. Test your water regularly to catch rising levels. See our how to cycle a fish tank and betta food guides for proper feeding practices.
Infrequent Water Changes
Dissolved organic compounds accumulate between water changes, providing a steady nutrient supply for algae. Without regular water changes, nitrate and phosphate have nowhere to go — algae takes over while plants struggle.
Overfeeding
The root cause of most nutrient problems. Feed only what fish consume in 2 minutes, once or twice daily. Remove uneaten food after feeding.
New Tank Syndrome
Biological filter not yet established means nitrate and phosphate are not processed efficiently. New tanks are prone to diatoms and green water. Choosing the right tank size helps — see our betta tank size guide.
Immediate Steps to Clear an Algae Bloom
Follow these steps in order for the fastest results.
Step 1 — Physically Remove as Much Algae as Possible
Scrape glass, vacuum substrate, prune affected leaves. Physical removal gives instant results and reduces nutrient load. The Aqueon Magnetic Aquarium Glass Cleaner (Medium) lets you clean from outside without wetting your hands.
Step 2 — Perform a Large Water Change
Change 30-50% of the water to dilute dissolved nutrients. Use a gravel vacuum to pull organic debris from the substrate. Match new water to tank temperature.
Step 3 — Use a UV Sterilizer for Green Water
UV light kills suspended algae cells as water passes through, clearing green water in 3-7 days without chemicals. The Fluval UV Clarifier handles tanks up to 100 gallons.
Step 4 — Apply Algae Control Treatment (If Needed)
For stubborn surface algae that keeps returning after manual removal, an algae control chemical can help. API ALGAEFIX Algae Control is safe for fish and plants when used as directed. Avoid using in tanks with invertebrates unless the product states it is safe.
Long-Term Algae Prevention
Clearing the current bloom isn’t enough on its own.
Control Light Duration and Intensity
Set lights on a timer: 8 hours on, 16 hours off. Don’t leave lights on longer — even an extra hour makes a difference. Replace bulbs every 12 months and avoid direct sunlight. See aquarium plants for planted tank lighting.
Maintain a Regular Cleaning and Water Change Schedule
Weekly 20-30% water changes with gravel vacuuming remove nutrients before they accumulate. Rinse filter media in tank water monthly. Test nitrate weekly — keep below 20 ppm.
Add Algae-Eating Tank Mates
Nerite snails for glass diatoms, Amano shrimp for hair algae, Otocinclus for soft green algae. Check betta tank mates for pairing advice, and species profiles: corydoras, neon tetra, guppy.
