How to Control and Prevent Brown Algae in Aquariums

In this guide

Brown algae, more accurately called diatoms, is one of the most common things a new aquarist runs into, usually within the first few weeks of setting up a tank. It looks alarming, a dusty brown film coating the glass, substrate and even plant leaves, but understanding what it actually is takes most of the worry out of dealing with it.

What diatoms actually are

Diatoms are single celled organisms with a distinctive glass-like cell wall built from silica, which gives brown algae its characteristic gritty texture when wiped, unlike the slippery feel of green algae or cyanobacteria. They need dissolved silicate to build these shells, and silicate is exactly the nutrient that is unusually abundant in a brand new tank, leaching from new glass, fresh substrate, and often the tap water itself.

Why it is so common in new tanks

A newly set up aquarium is close to the perfect diatom habitat: elevated silicate from new materials, elevated ammonia while the tank finishes cycling, and almost no competition from other organisms that would otherwise keep diatoms in check. This combination is why brown algae is considered close to a rite of passage in the hobby, appearing in the first two to three weeks of most new setups and fading naturally as the tank matures, often within a few months.

Removing it in the meantime

While waiting for a new tank to mature, brown algae is straightforward to manage physically:

• Wipe glass surfaces with an algae pad or scraper, since diatoms come off easily compared with more stubborn algae types
• Gently rub affected plant leaves between two fingers underwater to remove the film without damaging the leaf
• Vacuum the substrate during water changes to remove settled diatoms along with other debris
• Add algae-eating species such as otocinclus catfish or nerite snails, both of which graze diatoms readily

If it persists in an established tank

Diatoms that keep returning well after a tank has matured, typically past the six month mark, usually point to an ongoing silicate source rather than a normal new-tank phase. The tap water itself is a common culprit in some areas, since municipal water carries varying silicate levels depending on the source and treatment process. Switching to RO water for water changes, or blending it with tap water, is the most reliable long-term fix if source water testing confirms silicate is the issue.

Not a sign anything is wrong

It is worth repeating that brown algae in a new tank does not mean your water is dirty or your tap water is unusable, a misconception that leads some new keepers to unnecessarily invest in an RO system before they need one. Patience, regular light maintenance, and time for the tank to mature resolve the great majority of cases without any special intervention at all.

Telling it apart from cyanobacteria

Brown algae and slime algae are sometimes confused, since both can appear brownish, but the texture gives it away. Diatoms feel gritty and sandy when wiped between your fingers, a result of their silica shells, and generally need to be scraped or wiped off surfaces. Cyanobacteria feels slimy rather than gritty and often peels away in sheets on its own. Since the two have different causes and respond to different treatments, getting this distinction right before choosing a fix saves a lot of wasted effort.

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