Every tank gets algae. The goal isn't a tank with zero algae; that tank doesn't exist. The goal is a tank where algae stays in check well enough that you don't notice it. New keepers spend a lot of money on chemicals and cleanup-crew animals before learning that 90% of algae problems are solved by a 15-minute change to lighting or feeding.

This guide covers what causes algae, how to read which type you have, the fixes that work in order from least to most invasive, and the cleanup crews for both freshwater and saltwater. It applies to both setups; sections specific to one are labeled.

What feeds algae

Three inputs, in roughly this order of importance:

1. Light. Intensity and duration. Algae grows wherever a photosynthetic organism can survive, and algae outcompetes plants when light is abundant but other resources are scarce.
2. Nutrients. Nitrate and phosphate, usually from overfeeding, low water-change frequency, dead organic matter, or in saltwater, high-bioload mistakes.
3. CO2 imbalance (planted freshwater only). Plants that don't have enough CO2 can't compete with algae for nutrients. CO2 injection isn't required, but if you've added a lot of light without adding CO2 or limiting nutrients, algae wins.

Fix the inputs, the algae goes away. Add cleanup-crew animals before fixing the inputs, the cleanup crew can't keep up and you blame them for "not working."

Read the type before treating

Freshwater types

• Brown algae (diatoms). Soft, dusty, light-brown coating on glass, plants, and substrate. Common in new tanks (first 1-3 months) because silicates are leaching out of new substrate. Goes away on its own. Otocinclus and nerite snails will eat it through the worst of it.
• Green dust algae. Fine green film on glass that re-grows hours after wiping. Caused by excess light. Cut photoperiod first.
• Green spot algae. Hard, raised green dots on glass and slow-growing plant leaves (anubias, java fern). Phosphate deficiency in planted tanks (yes, deficiency). Counter-intuitive but real: dose phosphate (Seachem Flourish Phosphorus or potassium phosphate) and the spots stop forming.
• Hair / thread algae. Stringy green strands growing on plants and decor. Caused by high light + low CO2 (in planted tanks) or high nutrients + slow growth. Trim affected plants, manual removal weekly, and address the root cause.
• Black beard algae (BBA). Dark gray-purple tufts attached to driftwood, plant leaves, and hardscape. Loves low and inconsistent CO2, and high flow areas. Hard to remove physically; specific treatments below.
• Cyanobacteria (blue-green algae, BGA). Slimy, dark green-to-red sheet that lifts off in chunks and stinks. Not actually algae, it's bacteria. Caused by low flow, high organics, and low nitrate (in heavily-planted tanks). Specific treatment below.
• Green water. Free-floating single-cell algae that turns the water pea-soup green. Caused by direct sunlight or massive nutrient excess. UV sterilizer or 3-day blackout fixes it.
• Staghorn algae. Gray-green branching tufts that look like deer antlers. Less common, caused by ammonia spikes. Improving filtration and patience clears it.

Saltwater types

• Brown diatoms. Same as freshwater. Standard for new tanks (week 2 to month 3). Fades as silicates deplete.
• Hair algae (green hair algae, GHA). Bright green strands or carpet, common cause is rising phosphate. Remove rock, scrub the affected area, address phosphate at source.
• Bryopsis. Looks like hair algae but feathery. Very hard to kill; standard cleanup crews don't touch it. Specific treatments below.
• Cyanobacteria. Red, dark green, or maroon slime sheet. Caused by low flow combined with high organics. Same approach as freshwater.
• Dinoflagellates ("dinos"). Brown stringy snot with air bubbles trapped. Different from anything else listed and the hardest to treat in saltwater. Tied to ultra-low nutrient (UNL) tanks with nitrate or phosphate at zero. Counter-intuitive fix: dose nitrate and phosphate to bring them out of zero. Then chemical and physical treatments work better.
• Coralline algae. Pink, purple, red, or white encrusting algae on rock and glass. This is wanted. It's a sign of a healthy reef tank. Don't scrub it off rock; do scrape it off the front glass.

Step one: photoperiod and tank position

Almost every "I have an algae problem" can be partially fixed by:

1. Cut your light to 6-8 hours a day. Most algae outbreaks happen in tanks running 10-12 hours of light. Newer reefers and planted-tank keepers tend to want the tank lit when they're home, which is exactly the wrong instinct.
2. Move the tank off direct sunlight. Even an hour of direct sun a day will sustain an algae bloom no matter what you do otherwise. Curtains help, relocating helps more.
3. Reduce intensity, not just duration. LED fixtures usually have a percent-power setting. Running at 50% for 7 hours often beats 100% for 5 hours.

If your tank is in direct afternoon sun and lit for 11 hours by a top-of-the-line fixture, no cleanup crew or chemical will keep up. Fix the input first.

Step two: nutrients

Two levers:

• Feed less. Once a day, what the fish finish in 30-60 seconds. Most new keepers feed 2-3x too much.
• Water change more. 25-30% weekly is the standard. Most algae outbreaks coincide with skipped water changes.

In planted tanks, "less feeding" might actually mean "feed normally but vacuum the substrate weekly." Dead plant matter and fish waste accumulating in gravel feeds algae as it decomposes.

In saltwater, phosphate is the bigger lever:

• GFO (granular ferric oxide) in a media reactor pulls phosphate out of the water. Tested phosphate above 0.10 ppm? Run GFO.
• Carbon dosing (vodka, vinegar, NoPox, biopellets) lowers both nitrate and phosphate through enhanced bacterial uptake. Effective but easy to overdose into the opposite problem (dinos).
• Refugium with chaetomorpha. A macroalgae refugium on a reverse light cycle outcompetes nuisance algae for nutrients. Best long-term solution for nutrient control.

Step three: cleanup crew

Cleanup-crew animals work, but they're a maintenance tool, not a treatment. They can't fix a tank with the lighting or feeding wrong, but they keep a balanced tank looking clean.

Freshwater cleanup crew

• Amano shrimp (Caridina multidentata). Hair algae and biofilm, work harder than any other shrimp. 5-10 in a 20-gallon. Don't breed in freshwater so they don't overpopulate.
• Otocinclus. Brown diatoms and soft green algae. Group of 6 minimum. Won't survive in a too-new tank; wait 3+ months.
• Nerite snails. Green spot algae, brown diatoms, biofilm. Best glass cleaners in freshwater. Lay non-hatching eggs that can be unsightly.
• Bristlenose pleco. Glass and decor algae. See the pleco guide for the full breakdown.
• Siamese algae eater (Crossocheilus oblongus). One of the few species that eats black beard algae. Make sure you're getting the real species, not the look-alike Chinese or false SAE (which become aggressive adults).
• Mystery snail. Soft algae and detritus. See the apple snail guide for full care.

Saltwater cleanup crew

• Turbo snails (Turbo and Mexican Turbo). Heavy grazers, eat through hair algae faster than any other snail. Can knock over corals (clumsy at 2 inches).
• Cerith snails. Substrate sifters, eat detritus and film algae. Burrow during the day.
• Trochus snails. Glass cleaners, can right themselves if knocked over (turbo snails can't, which matters in a tank with strong flow).
• Blue-legged hermit crabs (Clibanarius tricolor). Eat detritus and film algae. Mildly aggressive toward snails (they take shells). Provide spare empty shells.
• Emerald crabs (Mithraculus sculptus). Eat bubble algae (Valonia) more reliably than anything else. Become opportunistic predators as they grow; one or two per tank.
• Sea hares (Aplysia species). Algae-eating slugs that consume hair algae at industrial scale. Buy when you have a hair algae outbreak, expect to rehome or sell after the algae is gone (they'll starve in a clean tank).
• Lawnmower blenny (Salarias fasciatus). Single fish per tank, scrapes film and hair algae. Personality bonus.
• Foxface rabbitfish (Siganus vulpinus). Larger algae eater, eats almost everything green. Tank size 75+. Venomous spines but rarely uses them.
• Yellow tang, kole tang, scopas tang. For 75+ gallon tanks. See the blue tang guide for the tang-size honest accounting.

A standard saltwater cleanup crew for a 40-gallon: 10 cerith snails, 5 trochus snails, 5 blue-legged hermits, 1 emerald crab.

Step four: targeted treatments

When inputs are fixed and the cleanup crew can't catch up, specific algae types respond to specific treatments.

Manual removal first

Always pull what you can by hand before treating. Turkey baster to blow detritus, toothbrush to scrub hardscape, tweezers for stubborn clumps. Manual removal during a 50% water change is the single most underrated treatment in the hobby.

Black beard algae (freshwater)

Liquid carbon / glutaraldehyde (Seachem Excel, Aquatic Arts Carbon). Spot-treat with a syringe directly on affected leaves, lights off, filter off for 10 minutes. Repeat daily for 1-2 weeks. Important caveat: vallisneria, anacharis, riccia, and some mosses die from glutaraldehyde. Don't use it in a tank with those plants.

Alternative: increase CO2 injection and flow. BBA struggles in tanks with consistent high CO2.

Hair algae (saltwater)

Hydrogen peroxide spot treatment. Remove rock from tank, spray 3% H2O2 directly on algae, wait 60 seconds, rinse and return. Kills algae on contact, safe for most corals if returned to tank promptly. For in-tank dosing, 1 mL of 3% H2O2 per gallon, dosed slowly over 30 minutes, with filter and lights on.

Bryopsis (saltwater)

High-magnesium dosing. Raise tank magnesium to 1700-1800 ppm using Tech-M (Brightwell or KEnt brands; the brand matters because some Tech-M formulations work, others don't). Hold for 4-6 weeks. Bryopsis recedes. Other algae and inverts mostly unaffected.

Cyanobacteria (both)

Black-out and flow. 3-day total darkness (cover the tank with a towel) plus a wave pump moving water across the affected area. Most cyano dies in 72 hours of darkness. Increase weekly water changes after.

Chemiclean (saltwater) or erythromycin (freshwater) as a fallback. Both kill cyano selectively. Both should be a last resort, not a first move. Improving flow and reducing organics permanently is the only durable fix.

Dinoflagellates (saltwater)

The hardest algae in the hobby to treat. The counter-intuitive fix is to raise nitrate and phosphate (to 5 ppm and 0.05 ppm respectively) by stopping all nutrient export (turn off GFO and skimmer for a week). Dinos thrive in zero-nutrient tanks. Once parameters are back up, dosing DinoX or running UV sterilization does the cleanup. Plan on 4-8 weeks.

Green water (freshwater)

UV sterilizer is the fastest fix. A small inline UV unit clears green water in 24-48 hours by killing the single-cell algae as it passes through. Alternative: 3-day blackout. Both work; UV is more controllable.

Last resorts

When everything above has failed and you have a tank you can no longer enjoy looking at:

• Full reboot. Tear down the tank, replace substrate, scrub rock and decor, restart the cycle. Drastic but sometimes the right call after months of fighting a stubborn outbreak.
• AlgaeFix and related polymer algicides. Kill algae across the board, but they're hard on inverts and unreliable around fish. Read the label and dose carefully. Many experienced keepers refuse to use these.
• Hire someone. Service companies will diagnose and clean tanks for $50-100. Sometimes the best money you spend on the hobby.

What experienced keepers know

A few things you only learn after a few algae cycles:

• Stable parameters beat aggressive chemistry. Tanks where you hold parameters steady for months grow less algae than tanks where you're constantly dosing.
• Algae is your tank communicating. Brown algae in month 2 means the tank is still settling. Green water out of nowhere means too much light or sun. Cyano in a low-flow corner means dead zones. Read the signal before throwing money at it.
• Plants compete with algae. Heavily planted freshwater tanks have less algae problems than fish-only tanks of the same size. Same for refugiums in saltwater.
• Don't blame the snail. Cleanup crews don't fix algae problems; they manage healthy tanks. If your nerites and ottos are "not working," it's the inputs that need fixing.
• Patience. Most outbreaks resolve themselves within 4-6 weeks if you cut light, cut feeding, increase water changes, and add an appropriate cleanup crew. Resist the urge to chemical-treat in week 1.

Putting it together

Use the Tank Builder to check bioload and stocking choices when you're rebuilding after a major algae crash; overstocking is one of the most common root causes. The compatibility and water-param checks will catch the cleanup crew species that don't suit your other fish.

Algae is the most-Googled aquarium topic for a reason. Almost every keeper hits a wall with one type or another. The good news is the wall is usually in week 8 of the tank's life, not at year 10. Get past the initial outbreak and the long-term tank stays clean almost by itself.