FishTankAquarium algae: types and fixes
Every tank grows some algae. A little is normal. A lot means the tank has more light or more nutrients than its plants can use. Work out which kind you have, then fix the cause.
The cause is almost always the same
Algae need light and nutrients, and nutrients come from fish waste and leftover food. Too much of either, and algae take over. Before you buy anything, check these:
- Light: 6–8 hours a day is plenty. A cheap outlet timer makes it automatic.
- Sunlight: keep the tank out of direct sun from a window.
- Feeding: only what your fish finish in about two minutes. See how often to feed fish.
- Water changes: about 25% a week removes the nutrients algae feed on.
- Plants: fast-growing and floating plants use up nutrients before algae can.
Which algae do you have?
Brown dust (diatoms)
A soft brown coating on glass, gravel and decorations, very common in tanks under a few months old. It wipes off easily and usually fades on its own as the tank matures. Nerite snails and otocinclus catfish eat it.
Green spots
Hard green dots on the glass that don't wipe off with a finger. Scrape them off with a magnetic cleaner or algae scraper, and cut back the light hours.
Green hair or thread algae
Long green strands on plants and decorations. Pull out as much as you can (twirling it around an old toothbrush works), cut the light hours, and feed less. Amano shrimp eat it.
Black beard algae
Dark, stiff tufts on plant edges, wood and filter parts. It's slow to clear. Trim off badly affected leaves, scrub decorations, and keep up water changes and good filter flow while you fix the light and feeding.
Blue-green slime
Slimy dark green or blue-green sheets that peel off and smell bad. It's actually a type of bacteria, not true algae. Siphon out as much as you can, do a water change, improve water flow, and feed less. If it keeps coming back, a "blackout" helps: turn off the light and cover the tank for three days, then do another water change.
Green water
Water that looks like pea soup is floating algae. See cloudy aquarium water for how to clear it.
Algae eaters that help
- Nerite snails: great on glass and decorations, and they can't breed in freshwater.
- Otocinclus catfish: tiny, peaceful algae grazers. Keep them in groups in an established tank.
- Amano shrimp: good on hair algae, but only with fish too small to eat them.
- Bristlenose plecos: stay small and graze algae. Avoid the "common pleco", which can grow over a foot long.
Algae eaters help, but they don't fix the cause. Without fixing light and feeding, the algae come back.
Helpful for beating algae
Paid links: if you buy through these Amazon links, I earn a small commission at no extra cost to you.
- Light timerBN-LINK 24-hour outlet timer (2 pack)Switches the light on and off at the same time every day, so it never runs too long.
- Glass cleanerMag-Float Mini magnetic cleanerA small magnetic scrubber that cleans the glass without getting your hands wet. Sized for small tanks.
- Gravel vacuumPython Pro-Clean, mediumRemove algae and waste from the gravel during water changes. The medium size suits tanks up to 20 gallons.
- Test kitAPI Freshwater Master Test KitHigh nitrate often goes with algae. Test it to know if you need bigger water changes.
Related: Cloudy aquarium water · How to do a water change · Best fish for beginners · Easy live plants