How to prevent aquarium algae (and clean it up fast when it happens)

Curated by the Fast Aquatics editorial team · Updated May 2026 · Sources

Algae blooms are a sign your tank is out of balance. Diatoms, hair algae, cyanobacteria, dinoflagellates - each has a different cause and a different fix. Here's the framework.

Diagnose by algae type

Brown diatoms. Common in new tanks (first 60-90 days). Caused by silicate from new substrate. Fix: time + cleanup crew + stable parameters.

Green hair algae. Excess nitrate + phosphate. Fix: water changes + reduce feeding + add macroalgae refugium.

Cyanobacteria (red slime). Stagnant flow + organic buildup. Fix: increase flow + 3-day blackout + Chemiclean or Red Slime Remover.

Dinoflagellates. Often sub-zero nitrate + zero phosphate (over-purified water). Fix: dose nitrate + phosphate to detectable levels, run UV sterilizer.

Green water (algae bloom). Direct sunlight or excessive lighting. Fix: blackout for 3-5 days + UV sterilizer.

Prevention checklist

Reduce feeding to 60-90 second consumption windows
Run lights 8-10 hours/day max
Weekly 25% water changes
Add cleanup crew matched to tank size (10 hermits + 10 nerite snails per 30 gallons)
Avoid placing tank in direct sunlight
Test phosphate + nitrate monthly - keep both detectable but low
Macroalgae refugium for reef tanks
Run carbon for organic export

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Frequently asked questions

Why does my tank get algae after every water change?

Likely tap water with elevated phosphate or silicate. Test your tap. Switch to RO/DI for reef. Use a phosphate-removing media (Rowaphos, Phosguard) inline.

How do I clean algae off glass?

Magnetic algae scraper for daily maintenance. For stubborn diatom film, razor blade scraper (acrylic-safe blade for acrylic tanks). Mr. Clean Magic Eraser works as a non-chemical scrub for new tanks.

Is algae bad for fish?

Most algae is harmless to fish. Cyanobacteria can release toxins in heavy blooms. Dinoflagellates are toxic to invertebrates and many fish. Both warrant aggressive treatment.