Guide
Brown algae in aquarium
Diatoms on glass and decor: common in new tanks and how to manage.
- What it actually is: Brown algae is almost always diatoms - single-celled organisms with silica-based shells, not true algae. The fix is different from green algae or cyanobacteria.
- Why it appeared: New substrate, tap water, and immature filtration supply the silicates and excess nutrients diatoms need. It's one of the most predictable events in a tank under six months old.
- Will it go away on its own? Usually yes. As silicates are consumed and your biological filter matures, diatoms typically fade over four to eight weeks without any intervention.
- What actually helps: Regular partial water changes, not overfeeding, consistent lighting (7-9 hours daily), and adequate flow. Manual wiping speeds things up cosmetically.
- What doesn't help: Cutting light to near zero, adding chemical treatments, or buying a cleanup crew as your first response. None of these address the root cause.
- When to dig deeper: If the brown coating is thick, slimy, and smells musty, test for cyanobacteria instead - the appearance is similar but the cause and fix are completely different.
What diatoms actually are
The brown film coating your glass, gravel, and plant leaves is almost certainly diatoms - a group of microscopic, single-celled organisms that sit somewhere between algae and their own kingdom (Bacillariophyta, if you want the taxonomy). What makes them distinct is their cell wall: a rigid shell made of silica called a frustule. That silica requirement is the key to understanding the whole diatom story in your tank.
They're not dangerous to your fish. They're not a sign your tank is failing. They are, however, a reliable sign that silicates and nutrients are available in quantity - which is almost always true in a new setup.
The brown film wipes off easily with a finger or algae pad, which helps distinguish diatoms from green spot algae (which resists pressure) and cyanobacteria (which comes off in sheets and often smells). Getting the identification right matters, because a treatment that helps one can be useless or harmful for another.
Most aquarium shops will sell you a product to "treat" brown algae. Save your money. Diatoms in a maturing tank are a phase, not a disease. Chemical treatments don't remove the silicate or nutrient supply driving the bloom - they just add more chemistry to a tank that's already in flux.
Why brown algae shows up in your tank
Diatoms need three things: silicates, light, and dissolved nutrients. New aquariums supply all three in abundance, which is why diatom blooms are so common in the first few months.
Silicates
Tap water is the main source. Depending on your local supply, silicate levels can be significant - many municipal water supplies run at 10-30 mg/L, and some run higher. New silica-based substrates (including many commercial sands and some gravels) also leach silicates as they settle in. This supply is finite; diatoms consume it, your filter media adsorbs some of it, and water changes dilute it. That's why blooms tend to be self-limiting.
Immature biological filtration
During and just after cycling your tank, the bacterial colonies converting ammonia to nitrite to nitrate are still establishing themselves. Ammonia and organic compounds are available in higher concentrations, giving diatoms a richer nutrient environment. As your nitrogen cycle stabilises, this advantage disappears.
Light and photoperiod
Diatoms are less light-hungry than many green algae, which means they can thrive even at lower intensities. Running your lights for inconsistent periods - or leaving them on all day - gives diatoms more opportunity without necessarily benefiting your plants enough to outcompete them.
Low plant mass
A well-planted tank pulls nutrients out of the water column continuously. Sparse planting leaves silicates, nitrates, and phosphates available for diatoms to use. This doesn't mean you need a dense planted setup - but it does explain why heavily planted tanks often see lighter diatom blooms.
Most diatom blooms in new tanks fade on their own within four to eight weeks as silicates are depleted and biological filtration matures.
What to actually do about it
The good news: your intervention list is short. Most of what works is just good husbandry you should be doing anyway.
Manual removal
Wipe the glass with an algae pad or magnetic scraper. Use a soft cloth or your fingers on plant leaves - diatoms come off without damaging foliage. For decor, a soft toothbrush works well outside the tank. Manual removal doesn't fix the underlying cause, but it keeps the tank looking presentable and removes a small amount of diatom biomass (and the nutrients locked in it) from the system.
Partial water changes
Regular water changes export dissolved silicates, nitrates, phosphates, and free-floating diatom cells. This is your most effective active tool. Aim for 20-30% weekly during a bloom - consistent smaller changes outperform infrequent large ones. See our guide on how often to do water changes for a more complete breakdown of scheduling.
Use a gravel vacuum when doing water changes during a diatom bloom. Diatoms settle into substrate gaps, and vacuuming the gravel removes both detritus and the diatoms feeding on it.
Feeding discipline
Uneaten food is a direct nutrient source for algae of all kinds. Feed only what your fish consume in two minutes or less, once or twice daily. Remove any uneaten food after five minutes. This single habit change has a bigger impact on long-term algae control than most products sold for the purpose.
Consistent lighting
Set a fixed photoperiod and stick to it - 7 to 9 hours per day is a reasonable range for most community tanks. Use a timer so it doesn't creep up. Keep the tank out of direct sunlight; window light is uncontrolled and can drive algae even when your artificial lighting is dialled in correctly.
Filtration flow
Check that your mechanical filter media isn't clogged. Reduced flow means reduced oxygen, reduced bacterial activity, and elevated organic waste in the water - all of which encourage algae. Rinse mechanical media in old tank water (never tap water) when it becomes visibly restricted.
Turning off the lights for several days to "starve" diatoms. This rarely works - diatoms tolerate low light well - and it stresses your plants, which are your best long-term competition against algae. Keep a consistent photoperiod instead.
Cleanup crew: helpful, not a cure
Certain tank inhabitants will graze diatoms from surfaces, and they're worth adding if they suit your setup. The honest caveat: they manage the symptom. If silicates and nutrients remain high, diatoms will return faster than any cleanup crew can keep up.
Useful grazers include:
- Otocinclus catfish (Otocinclus spp.) - small, peaceful, excellent diatom grazers on glass and broad leaves. Keep in groups of six or more; they're shoaling fish and decline alone.
- Nerite snails (Neritina spp.) - efficient, don't reproduce in freshwater, and won't overpopulate. One or two per 40 litres is plenty.
- Amano shrimp (Caridina multidentata) - general algae and detritus eaters, effective in groups of five or more.
- Bristlenose plecos (Ancistrus spp.) - stay manageable in size (10-15 cm), good on glass and decor. Avoid common plecos (Hypostomus plecostomus) in most home tanks - they reach 50+ cm.
If you're planning your stocking and want to make sure these additions fit within your tank's capacity, run the numbers through the free stocking calculator before buying.
Otocinclus are sensitive fish that ship poorly and often arrive stressed. Quarantine them before adding to your display tank, and don't add them to an uncycled or unstable setup. Check your water parameters are stable first: ammonia and nitrite at zero, pH between 6.5-7.5, temperature 22-26°C (72-79°F).
What most guides miss
Silicate test kits
Most fishkeepers never test for silicates, and most basic test kits don't include it. If you have a persistent diatom problem - one that doesn't resolve after two to three months of good husbandry - it's worth testing your tap water for silicates. If levels are high (above 30 mg/L), consider using a reverse osmosis unit or remineralised RO water, at least for top-ups. This is a niche step that the majority of hobbyists will never need, but it exists as an option if standard approaches aren't working.
Diatoms vs cyanobacteria: get the ID right
Cyanobacteria (blue-green algae) is often mistaken for brown diatoms, especially early on. The differences: cyanobacteria typically smells earthy or musty, forms thicker mats that peel off in sheets rather than wiping away easily, and can appear blue-green, dark brown, or almost black. It's driven primarily by low flow and high nutrients - not silicates - and the treatment approach is different. If you're unsure, the smell test is the fastest diagnostic.
New tank syndrome isn't a failure
Diatom blooms are so consistent in new tanks that some experienced aquarists consider them a sign the cycle is progressing normally. The bacteria processing ammonia produce byproducts that diatoms can use; as those bacteria establish fully and the chemistry stabilises, the diatoms lose their competitive edge. If your ammonia and nitrite are both reading zero and the tank has been running for a few weeks, a diatom bloom alongside that is a normal picture - not a problem requiring intervention.
The single biggest predictor of how long your diatom bloom lasts is patience combined with consistent water changes. Tanks that get 20-25% weekly changes during the first two months almost always clear up faster than tanks where owners add products, cut lights, and change their approach every week trying to solve it faster.
Monitoring your tank through a diatom phase
During and after a bloom, keep testing regularly: ammonia, nitrite, nitrate, and pH at minimum. Logging these over time shows you whether the tank is genuinely maturing or whether something else is driving elevated nutrients. App-aquatic lets you log water test results with timestamps and flags parameter trends over time - useful when you're trying to see whether things are actually improving week over week or just fluctuating.
Target parameters during a diatom bloom (and after): ammonia 0 mg/L, nitrite 0 mg/L, nitrate below 20 mg/L for most community fish, pH 6.5-7.5 for most species. Full reference ranges are in our aquarium water parameters guide.
Track your parameters and get care reminders with App-aquatic.
Get the free appIs brown algae harmful to my fish?
No. Diatoms don't harm fish directly, don't deplete oxygen at levels typical in a home aquarium, and aren't toxic. Heavy coating on plant leaves can reduce photosynthesis, but this is rarely severe enough to cause plant loss in the timeframe of a normal bloom.
My tank is six months old and still has brown algae - why hasn't it gone away?
A few possibilities: your tap water may have persistently high silicates, you may be overfeeding, your lighting schedule may be inconsistent, or your filtration may be undersized for your stocking level. Test your tap water for silicates, review your feeding habits, and check that your filter is rated for at least the volume of your tank. If nitrate is consistently above 20-40 mg/L, increase your water change frequency.
Will adding more plants help?
Yes, meaningfully so. Fast-growing stem plants like Egeria densa, Vallisneria, or floating plants like frogbit compete directly with diatoms for nutrients. They won't eliminate a bloom overnight, but a well-planted tank consistently shows lighter algae issues long-term. Make sure lighting is adequate for plant growth if you go this route.
Should I do a blackout (no light for several days) to kill the diatoms?
Blackouts are used for cyanobacteria and certain green algae - not diatoms. Diatoms tolerate low light better than your plants do, so a blackout is more likely to set back your plant growth than resolve the diatom bloom. Stick to a consistent 7-9 hour photoperiod.
Can I use a UV steriliser to get rid of brown algae?
A UV steriliser will kill free-floating diatom cells passing through the unit, which can reduce the density of the bloom. It won't remove diatoms already attached to surfaces, and it won't address the silicate or nutrient supply feeding the bloom. It's a useful tool in a mature, well-maintained tank - but buying one specifically to deal with new-tank diatoms is solving a temporary problem with a permanent (and expensive) piece of equipment.
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