Grey Algae in Your Aquarium: What It Is and How to Remove It

Hektor Jorgo By Hektor Jorgo Published Updated 4 min read
A thin grey diatom film coating the glass and decor of a recently set up aquarium

Quick Facts

What It Usually Is
A thin grey or brownish film, most often diatoms — especially common in new tanks
Most Common Cause
New tank cycling (diatoms feeding on silicates) or low-light, low-flow areas in older tanks
Texture
Soft, dusty film that wipes off easily — unlike tougher black beard algae
New Tank Timeline
Often appears within the first 1-4 weeks and fades on its own as the tank matures
Established Tank Grey Film
Usually points to a specific low-light/low-flow spot rather than a tank-wide problem
Algae Eaters
Nerite snails and otocinclus are commonly used for grey film/diatom control
Wiping vs. Fixing
Wiping addresses the symptom; repeated regrowth in the same spot means addressing light or flow there
Not the Same As
Green spot algae (GSA) or black beard algae (BBA), which need different approaches

A thin grey or brownish haze creeping across the glass, decor, and substrate of a relatively new aquarium is one of the most universal sights in the hobby — and also one of the most likely to prompt a worried forum post from a first-time tank owner. The good news: in the vast majority of cases, grey algae is diatoms, it's normal, and it tends to resolve itself without much intervention.

Direct Answer: Usually Diatoms, Usually Temporary

"Grey algae" is most commonly diatom algae — a soft, dusty film, typically grey to light brown, that wipes off easily with a finger, algae pad, or scraper. Diatoms are quick to colonize bare surfaces, which is why they're so often the first visible algae in a brand-new aquarium, frequently appearing within the first 1-4 weeks of setup. In most cases, this film fades on its own over the following weeks as the tank's biological community matures — no special treatment is typically needed beyond the cleaning you'd be doing anyway.

This is different from algae types covered in our general algae guide like green spot algae (hard, circular, green) or black beard algae (tough, dark, hair-like) — both of which have a different texture and don't follow the same "fades with time" pattern.

Why New Tanks Get a Grey Film

When a tank is first set up, every surface — glass, decor, substrate, plant leaves — is essentially a blank slate. Diatoms are efficient early colonizers of these surfaces, partly because they can use silicates (often present in tap water and some substrates) that other algae and plants don't rely on as heavily. As the tank matures — plants establish, other organisms compete for resources, and silicate levels in the water column may also drop — diatoms typically decline on their own. Our guide on how long algae takes to grow covers this new-tank timeline in more detail, including how it compares to algae blooms in established tanks.

Established Tanks: Grey Film in Specific Spots

If an otherwise mature, algae-light tank develops a grey film that's concentrated in one particular area — rather than appearing tank-wide — that's usually less about "the tank cycling" and more about local conditions in that spot: lower light (a shaded corner), or lower flow (behind decor, in a dead zone). The fix in this case is more targeted than "wait it out" — addressing light or flow to that specific area, similar to the dead-spot considerations discussed in our Calothrix algae guide for cyanobacteria, tends to resolve a persistent localized film better than repeated wiping alone.

Removal and Algae-Eater Options

  • Manual wiping/scraping — straightforward and effective for diatom film; an algae pad or scraper handles glass, and a soft brush or thumb works for decor and slow-growing plant leaves like java fern, which our java fern algae guide covers as a case where slow leaf turnover means film algae tends to linger longer than on faster-growing plants.
  • Nerite snails — widely used specifically for film algae and diatoms, and don't reproduce in freshwater, making population management simple.
  • Otocinclus catfish — graze on soft algae films including diatoms, though they're a community fish with their own care requirements (groups, established tanks) rather than a "drop in and forget" solution. Both of these stay relatively predictable in size, which our guide to algae-eater growth rates covers in more detail compared to faster-growing species.
  • Patience for new tanks — for a brand-new setup, routine wiping during normal maintenance combined with time is often all that's needed; the film typically declines without further action.

Quick Reference

  • Grey algae is most often diatoms — a soft film that wipes off easily
  • Common and normal in new tanks during the first 1-4 weeks
  • Usually fades on its own as the tank matures, without special treatment
  • Persistent grey film in one spot in an established tank points to local light/flow issues
  • Nerite snails and otocinclus are commonly used for film algae/diatom control
  • Different from green spot algae or black beard algae, which need different approaches
  • For new tanks, routine wiping + time is usually sufficient

Frequently Asked Questions

Is grey algae on my new tank's glass or decor normal?

Yes, very. A thin grey-brown film appearing within the first few weeks of a new tank is one of the most common and normal experiences in the hobby — it's almost always diatoms, a type of algae that's quick to colonize the bare, uncolonized surfaces of a new aquarium. It tends to fade on its own over the following weeks as the tank's biology matures, even without specific intervention beyond routine cleaning. Our algae growth timeline guide covers what to expect during this period in more detail.

How do I get rid of grey algae?

For new-tank diatom film, the most practical approach is routine wiping/scraping during regular maintenance, combined with patience — it generally fades within a few weeks regardless of cleaning frequency, since it's tied to the tank's overall maturation rather than any single fixable cause. For grey film that persists or reappears in a specific spot in an established tank, the more durable fix is to look at light and flow in that exact location — a shaded corner or a dead-flow area behind decor is a common pattern, similar to the dead-spot causes covered in our Calothrix algae guide for cyanobacteria.

Will grey algae harm my fish or plants?

Generally no — grey diatom film is not toxic and doesn't pose a direct threat to fish. It can be cosmetically unwelcome and, if it builds up heavily on plant leaves, can reduce the light reaching the leaf surface, which matters more for slow-growing plants. For most new tank owners, grey algae is more of a 'this is normal, it'll pass' situation than something requiring active treatment, beyond routine wiping if it bothers you visually.

What's the difference between grey algae and brown algae?

In practice, 'grey algae' and 'brown algae' often describe the same thing — diatoms — with the exact shade varying based on lighting, substrate color, and the specific diatom species present. Both share the same soft, dusty texture that wipes off easily, the same new-tank prevalence, and the same general tendency to fade over time. The naming difference is mostly about how it looks under your particular tank's lighting rather than a meaningfully different type of growth. Our broader algae guide covers how diatoms compare to other algae types like green spot algae and black beard algae, which have a noticeably different texture and don't fade the same way.

Sources & Further Reading

  1. Algae Control in the Aquarium — Practical Fishkeeping
  2. Planted Tank Algae Discussion — The Planted Tank Forum
Hektor Jorgo

About the Author: Hektor Jorgo

Co-Founder & Marine Biologist

Hektor is a co-founder of Sea Life Planet and has kept reef and freshwater aquariums for over 15 years. He holds a background in marine biology and focuses on species care accuracy, water chemistry, and tank husbandry.

Related Articles

Green algae growth on the glass and decor of a planted freshwater aquarium
Aquarium Plants & Algae

Algae in Aquariums: Types, Causes, and How to Get It Under Control

Nearly every aquarium develops algae at some point — the difference between a tank with a passing algae phase and one with a chronic problem usually comes down to light and nutrients, not luck.
A thin brown algae film developing on the glass of a newly set up aquarium
Aquarium Plants & Algae

How Long Does It Take for Algae to Grow in an Aquarium?

New tank owners often expect algae to be a distant problem — but depending on the type, algae can show up within days, and a sudden bloom in an older tank usually has a recent cause.
Dark tufted cyanobacteria growth on aquarium hardscape, often confused with black beard algae
Aquarium Plants & Algae

Calothrix Algae: Identification and How to Get Rid of It

Calothrix forms dark, hair-like tufts that look a lot like black beard algae — but it's cyanobacteria, not true algae, and that distinction changes how you treat it.
An anubias plant with its rhizome mounted on driftwood above the substrate
Aquarium Plants & Algae

Anubias Rot: Why It Happens and How to Stop It

Anubias is one of the toughest aquarium plants, but rhizome rot — usually caused by burying the rhizome in substrate — is the main way it fails, and it's preventable.
A sprig of hornwort with green new growth at the tips and browning needles lower down
Aquarium Plants & Algae

Hornwort Turning Brown: Causes and How to Fix It

New hornwort dropping brown needles right after purchase is one of the most common 'is my plant dying' moments in the hobby — and usually isn't.
A male American flagfish displaying its red and blue-green coloration in a planted aquarium
Freshwater Fish

American Flagfish Care Guide: The Algae-Eater With a Temper

American flagfish are one of the few fish that reliably eat hair and beard algae — but their territorial temperament means they're not the drop-in-anywhere community fish their small size suggests.