Brown deposit in my aquarium, what should I do?

A powdery brown deposit on the glass, rocks, and plants is most often a film of diatoms, also known as brown algae. It wipes off, is common in a new tank, and does not harm the fish. Wipe it off, change 20 to 25% of the water, and test for silicates if the film returns. If the deposit is slimy, comes off in sheets, or smells bad, it is probably something else: the test below will help you tell.

Recognize the deposit in one minute

The right reflex is not to look for a product, but to touch. Take a little deposit from a glass or rock and rub it between two fingers. The test proposed by JBL is simple: rough, it is diatoms; greasy, it is cyanobacteria.

Color alone is not enough. Diatoms range from brown to mustard or rust-colored. Cyanobacteria are often blue-green, but they can also be brown, red, or black. They form a slimy film and give off a musty or swampy odor, while diatoms have almost none.

The deposit can also be a simple accumulation of waste: food scraps, excrement, and plant fragments form brown flakes that rise when you pass your hand over them. They do not stick to the glass and are mostly found in corners and behind decorations.

Why diatoms appear

Diatoms are tiny organisms that make a silica shell. They therefore need dissolved silicates in the water. JBL indicates that they can form their shell and cover the aquarium above 0.8 mg/l. GreenAqua considers that a value below 1 mg/l is ideal and that action should be taken above 2 mg/l.

These silicates come from tap water, whose content varies by region, as well as from certain sands and gravels, volcanic rocks, and ceramics. New tanks are the most affected: the filter is not yet seasoned and the biological balance is being established.

It is often said that low lighting is the cause. GreenAqua cites it, but Petco specifies that this link has not been proven. Increasing the light without other corrections is therefore not the first step to try, especially since it can promote other algae.

Clean without unbalancing the tank

1. Glass. Use a magnetic cleaner or a scraper, from the bottom up, rinsing the tool regularly in chlorine-free water.
2. Plants. Gently wipe the leaves between two fingers or with a clean sponge. Our article on algae on aquarium plants details the process.
3. Decorations. Remove them and brush them in the water removed from the tank, without soap or detergent.
4. Bottom. Pass the siphon over the brown patches, just above the sand to avoid sucking it up.
5. Water. Change 20 to 25% of the volume with conditioned water at the same temperature, following our guide how to change the water in an aquarium.
6. Filter. Rinse the saturated sponges in the removed water, never under the tap, to preserve the bacteria.

Limit the return of the brown film

In a new tank, a little patience and regular maintenance are often enough: the film recedes when the whole thing stabilizes. In an old tank, a repeated return deserves a measurement.

• Measure the silicates in the tank and tap water. If the fill water is heavily loaded, every water change brings some in.
• Use silicate filter media if the value is high. JBL recommends it for continuous use when tap water is rich in silicates. Reverse osmosis water is another solution.
• Rinse well sand and gravel before filling a new tank.
• Dose the food to limit available nutrients.
• Add fast-growing plants, which consume some of these nutrients.
• Think about grazers if your tank is suitable: Amano shrimp, nerite snails, or otocinclus consume this film. They help, but they do not correct the cause.

If it is not diatoms

Cyanobacteria. They benefit from an excess of nutrients, low flow, and a heavily loaded bottom. Remove them by hand or with a siphon, increase the flow in dead zones, reduce food, and clean the bottom. Grazers do not eat them. Our guide for treating cyanobacteria in aquariums goes further.

Accumulated waste. Siphon, rinse the filter, and review the amount of food. Our article how to clean the bottom of an aquarium offers several methods.

Brown water rather than deposit. If it is the water itself that is colored, without a film on the glass, look toward the wood or leaves, which release tannins. This is not the same problem.

Frequently asked questions

Is the brown deposit dangerous for my fish?

Diatoms do not harm fish. On the other hand, a thick layer covers the leaves and hinders the plants. If the deposit is slimy and smells bad, think rather of cyanobacteria, which require a different approach.

How long do diatoms last in a new aquarium?

There is no fixed duration. They often recede when the tank stabilizes, provided you change the water and maintain the filter. If they persist in an old tank, measure the silicates in the tap water.

Should I cut the light?

This is not a lasting solution: prolonged darkness stresses the fish and hinders the plants. The link between low light and diatoms is also not proven. Keep a regular lighting duration.

Why does the deposit return after cleaning?

Because cleaning removes the film, not its source. As long as the water brings silicates or the tank remains loaded with waste, it can reappear. Measure, then correct the cause.

Also read

• I have algae in my aquarium, what to do?
• How to clean algae in an aquarium?
• Which aquarium test kit to choose?

Last updated: November 4, 2026.