Brown Algae in a New Tank: Normal and How to Remove

Is brown algae in a new tank normal?

Yes, brown algae in a new aquarium is completely normal and is one of the most predictable events in fishkeeping. What you are seeing is almost certainly a diatom bloom, and it resolves on its own in most cases within four to eight weeks as your tank matures.

Diatoms are single-celled organisms with rigid, glass-like cell walls made of silica. They are not true algae in the strict biological sense, but they behave like algae from a practical standpoint, forming a dusty, yellow-brown to rusty-brown film across your glass, substrate, rocks, and plant leaves. If your tank is less than six months old and you are looking at a dry, powdery brown coating that seems to have appeared out of nowhere, diatoms are the near-certain explanation.

What causes diatoms in a new tank?

Diatoms need three things to proliferate: dissolved silicate (to build their glass cell walls), dissolved nutrients such as ammonia, nitrate, and phosphate, and light. A new aquarium delivers all three simultaneously, which is why the bloom happens so reliably.

Dissolved silicate enters your water primarily through your tap supply. Municipal tap water commonly contains 10 to 30 mg/L of silicate, and water treatment does not remove it because silicate poses no concern for human health. New silica-based substrates such as sand and certain gravels also leach silicate directly into the water column, adding to what your tap already supplies.

On the nutrient side, the timing matters. During and just after cycling, the bacterial colonies that convert ammonia to nitrite and then to nitrate are still getting established. Ammonia and organic compounds are available at higher concentrations during this window, giving diatoms a richer feeding environment. As the nitrogen cycle stabilizes and beneficial bacteria compete more effectively for those nutrients, the diatom bloom loses its advantage and naturally fades.

Light rounds out the picture. Diatoms are less light-hungry than many green algae and can thrive even at low intensities. Running your lights inconsistently or leaving them on for long, unstructured periods gives diatoms more opportunity without providing the competing plants enough structure to push back.

How to tell if you actually have diatoms (and not something worse)

Before treating anything, confirm you are dealing with diatoms and not cyanobacteria or dinoflagellates, which look similar but require completely different responses.

Diatoms form a dry, dusty, or powdery light-brown film. Blow on a patch with a turkey baster and it disperses into a brownish cloud in the water, settles quickly, and does not reorganize. There is no odor.

Cyanobacteria (blue-green algae) form thicker, slimy sheets. They stay matted together when agitated and carry a distinct earthy or foul odor that diatoms completely lack. In freshwater they tend to appear blue-green, teal, or dark green. In saltwater they often appear reddish-brown or dark maroon. Cyanobacteria do not self-resolve reliably and can smother plants and, during severe blooms or die-offs, reduce dissolved oxygen to levels that stress fish.

Dinoflagellates form a sticky, mucous-like, often bubbly brown or golden slime that is harder to blow off than diatoms. When agitated and passed through a fine filter, the material passes through almost completely. After sitting under ambient light for roughly an hour, it reassembles into colonies. Like diatoms, dinoflagellate films recede noticeably when the lights go out, which helps distinguish both of them from cyanobacteria. Importantly, some dinoflagellate species produce toxins capable of killing corals, invertebrates, and fish, making correct identification genuinely important before you do anything.

If the film is dry and dusty, blows off easily, and carries no odor, you almost certainly have diatoms. Proceed accordingly.

What to do about diatoms in a new tank

Manual removal. Diatoms do not attach firmly to surfaces, so physical removal is straightforward. On glass, wipe them off with an algae scraper, an algae magnet, or even a credit card. On plant leaves, brush them off gently with a finger or a soft toothbrush. On rocks and hardscape, a firm toothbrush works well. Manual removal is cosmetic and does not address the underlying bloom, but it keeps your tank presentable while the biological filter finishes maturing.

Fix your photoperiod. If your light schedule is irregular, set it to a consistent seven to nine hours per day. Structure gives your plants a competitive edge and avoids giving diatoms unearned extra time under light.

Be patient. As the silicate supply from substrate leaching is consumed and your biological filter matures, diatoms typically fade on their own over four to eight weeks. Other algae, beneficial bacteria, and algivorous organisms then compete successfully for the remaining resources. The single most effective thing you can do in a new tank is give the cycle time to complete.

Add cleanup crew (with timing in mind). For freshwater tanks, nerite snails are excellent at scraping diatoms from glass and hard surfaces using their specialized feeding structure. Amano shrimp graze diatoms from plants and hardscape effectively. Otocinclus catfish specialize in soft algae and diatoms on glass and leaves, but it is worth noting that they are a delicate species that does best in mature, stable water chemistry. Adding them during active cycling, when ammonia and nitrite spikes are possible, carries real risk. Hillstream loaches are another option for glass and rocks in higher-flow setups. For marine tanks, many hobbyists report nerite snails and certain turbo-style snails working through diatom films efficiently.

What to do if diatoms persist past two to three months

Diatoms that do not resolve after two to three months of good husbandry suggest that silicate is being continuously replenished faster than natural processes can consume it. The most common culprit is high-silicate tap water restocking the supply with every water change. Testing your tap water for silicate is a sensible next step in this situation. If your readings are high, some hobbyists use reverse osmosis filtered water or a silicate-removing resin to reduce the incoming silicate load, though the threshold at which this becomes necessary is not agreed upon across sources and the right call depends on your specific readings.

Does brown algae harm fish or invertebrates?

No. Diatoms are not harmful to fish, shrimp, invertebrates, or corals at the bloom concentrations typical of new aquariums. They are unsightly but pose no toxicity risk. Silicate itself causes no direct harm to fish or invertebrates at levels found in home aquariums. The concern with silicate is only what it feeds, not what it does directly to your animals.

Frequently asked questions

How long does the brown algae phase last in a new tank?

In most cases, a diatom bloom in a new aquarium fades over roughly four to eight weeks as the nitrogen cycle stabilizes and the silicate supply from substrate leaching is consumed. Some tanks clear faster, and some take longer depending on tap water silicate levels and how quickly the biological filter matures.

Will brown algae go away on its own?

Yes, diatom blooms are self-limiting and resolve on their own in most new tanks without intervention. As the nutrient advantage diatoms enjoy during cycling disappears and competing organisms establish themselves, the bloom fades naturally. Manual removal and a consistent light schedule help in the meantime, but they are not strictly necessary.

Is brown algae dangerous to my fish?

No. Diatoms at typical new-tank bloom concentrations are not toxic and pose no direct risk to fish, shrimp, invertebrates, or corals. The concern with brown algae is cosmetic, not medical.

Can I add Otocinclus catfish to eat the brown algae?

Otocinclus are effective grazers of diatoms on glass and plant leaves, but they are a sensitive species that does best in established tanks with stable water parameters. Adding them during active cycling, when ammonia and nitrite can spike, carries meaningful risk to the fish. It is safer to wait until your tank has fully cycled before introducing them.

What if my brown algae does not look dusty, or it smells bad?

If the film is slimy and matted rather than dry and powdery, or if it carries a distinct earthy or foul odor, you may be dealing with cyanobacteria rather than diatoms. Cyanobacteria do not self-resolve reliably and require a different approach, including addressing nutrients and circulation. Check the identification guide above and, if you are unsure, test your water before acting.

If you want a faster answer, drop your actual water readings into TankMind's /diagnose tool and it will help you pinpoint which organism you are dealing with and what your numbers suggest as the cause.

Want this pinned down for your own tank? Get a free diagnosis from your actual water readings.