Brown Algae (Diatoms) in a New Tank: Why It Shows Up

The short version

• Brown algae is a bloom of diatoms, and it is the single most common thing to appear in a tank under two to three months old.
• It feeds on silicates and the loose nutrients of a system that has not settled, and it favors lower light.
• In most new tanks it peaks around week two to four, then fades on its own as the tank matures and silicates run down.
• A cleanup crew handles the rest: a nerite snail, amano shrimp, or an otocinclus in a mature tank grazes it off glass and hardscape.
• If it is still spreading after three months, something is feeding it: silicate-heavy source water, too little light, or a high nutrient load.

Brown algae looks alarming the first time it coats a fresh tank: a dull rust-brown dust over the sand, the glass, and the leaves of your slowest plants. Wipe the glass and it smears rather than scrubs, and it is back in a day or two. That is the tell that you are looking at diatoms, not green algae, and in a new tank it is a phase almost every keeper passes through.

Diatoms are single-celled organisms that build glassy shells out of dissolved silica. A brand-new tank hands them everything they want: silicates leaching from new substrate and tap water, nutrients that no established plant mass or bacteria colony is using yet, and often a modest light level. None of that is a disease, and none of it means you did something wrong. Here is what is actually happening, and how to move the tank past it.

What brown algae actually is

Brown algae is not a plant and not a true algae in the way green spot or hair algae are: it is a colony of diatoms, microscopic organisms that pull silica out of the water to build their shells. That is why it looks and behaves differently from green algae. It forms a flat, dusty film rather than strands or spots, and it lifts off in a brown cloud when you disturb it.

The reason it dominates a new tank and not an old one is supply. A tank in its first 2 to 8 weeks is releasing silicates from fresh substrate and carries whatever silica your tap water holds, while nothing else is competing hard for nutrients yet. Diatoms are fast to exploit that gap. As plants root in and the bacteria colony matures, the easy silica and surplus nutrients get used up, and the diatoms lose their advantage.

Why it blooms in a new tank

The timing lines up with the nitrogen cycle. During the first month, ammonia and nitrite are still converting and plants have not yet hit full growth, so nutrients sit in the water unused. Diatoms move into that window. A liquid test kit that reads ammonia, nitrite, nitrate, and pH is the tool that tells you where the cycle is, and a brown bloom usually shows up somewhere in the first 2 to 4 weeks alongside those shifting numbers.

Silica is the other half. New sand, some rock, and a lot of municipal tap water carry dissolved silicates, and diatoms build their shells from exactly that. This is why two tanks set up the same week can bloom differently: the one filled with silicate-rich water browns harder. It is also why the bloom fades. Once the initial silica is spent and plants are pulling nutrients, the diatoms starve back on their own, usually by month 2 or 3.

The design causes, and the fix for each

Most brown algae needs no product, just time and a few adjustments. Match the cause to the fix:

What is feeding it	What to change
A new tank still cycling	Wait it out: most blooms fade by month 2 to 3 as the system matures
Silicate-rich tap water	Consider RO water or a silicate-absorbing media if it persists past 3 months
Light too low or too short	Steady the photoperiod at 6 to 8 hours so plants can win
Surplus nutrients, low plant mass	Add fast plants to out-compete it, and do not overfeed

Light is the one people get backwards. Diatoms tolerate dim tanks better than most plants do, so a weak light can hand them the edge. A dimmable fixture such as a full-spectrum LED bar set for roughly 30 to 50 PAR at the substrate, on a timer for 6 to 8 hours, favors plant growth over diatoms. Turning the light off entirely does not help, because it starves your plants faster than the diatoms.

Who eats brown algae

Diatoms are one of the few algae types that a cleanup crew genuinely clears, because they are soft and easy to graze. The nerite snail (Neritina sp.) is the standout: it stays around 1 inch, never eats healthy plants, and works glass and hardscape all day, though it wants harder water (GH 6 and up) to keep its shell. It will polish a diatom film off the glass faster than you can wipe it.

Amano shrimp (Caridina multidentata), at about 2 inches in a small group, also graze diatoms steadily. An otocinclus (Otocinclus sp.) is the classic diatom-eater, but with a catch: it needs a mature tank of three months or more with a steady biofilm supply, and it should go in as a group of six or more, so it is a poor choice for the brand-new tank where brown algae first appears. Add otos after the tank has aged, not during the first bloom.

The honest part: timelines and effort

Brown algae is a waiting game more than a fight. In a typical new tank the bloom builds over the first two to four weeks, holds for a while, and clears within two to three months as the tank finds its balance. Scrubbing it away daily does nothing to the cause, and a big water change during cycling can even stretch the process out by resetting the tank's progress.

What you actually do in the meantime is small. Wipe the front glass before it bothers you, keep the light steady at 6 to 8 hours, feed lightly, and let plants and time do the work. If the bloom is still spreading past the three-month mark, read that as a signal of a standing cause, most often silicate-heavy source water or too little light, and change that one thing rather than reaching for a chemical.

Frequently asked questions

Is brown algae bad for fish or shrimp?

No. Diatoms are harmless to fish, shrimp, and snails, and shrimp will even graze on the film. It is an appearance problem and a sign of a young tank, not a health threat. That said, if an animal looks unwell, the algae is not the reason to look closer, the animal is.

How long does brown algae last in a new tank?

In most tanks it clears within two to three months as the system matures and silicates deplete. The visible peak is often around week two to four. If it is still spreading after three months, a standing cause is feeding it and worth tracking down.

Will more light get rid of brown algae?

Steadier, adequate light helps, because it lets plants out-compete diatoms; total darkness does not, because it starves plants first. Aim for a consistent 6 to 8 hour photoperiod rather than a long or erratic one. In a plant tank, healthy plant growth is what ends a diatom bloom for good.

What is the difference between brown algae and green algae?

Brown algae is a diatom film that wipes off in a dusty smear and favors new tanks and low light. Green algae (spot, hair, or a green-water bloom) is a true algae driven by light and nutrients in an established tank. They look different and are handled differently.

Brown algae is the tank telling you it is still young. Give it a cleanup crew suited to the tank's age, hold your light and feeding steady, and let it age out. To plan a stocking list and a plant mass that keeps a new tank ahead of algae from the start, run the setup through the build planner, or pick a grazer for your water from the compatibility database. If the trouble on your plants looks more like dissolving leaves than a film, read why aquarium plants melt next, or compare it against black beard algae and a green water bloom in the troubleshooting library.