Problem Solving
Brown Algae in Fish Tanks: Causes, Removal & Prevention Guide
Brown algae coating your tank glass or gravel? Learn exactly what causes it, how to remove it fast, and how to stop it from coming back — with or without chemicals.
Brown algae is the dusty brown coating that shows up on your glass, gravel, decorations, and plant leaves -- usually within the first few weeks of setting up a new tank. It looks awful, it smears when you touch it, and it makes you wonder what you did wrong. The short answer: probably nothing. That brown film is almost certainly diatoms, and they're one of the most common and predictable problems in freshwater aquariums.
This guide covers what diatoms actually are, why they colonize your tank, whether they're dangerous, and exactly how to remove them and keep them from coming back.
What Is Brown Algae? (It's Not Actually Algae)#
Despite the name, brown algae in aquariums isn't true algae at all. It's a colony of diatoms -- microscopic single-celled organisms with cell walls made of silica (the same compound in glass and sand). They photosynthesize like plants, but their biology is fundamentally different from the green algae, black beard algae, or cyanobacteria you might also encounter.
Diatoms Explained -- What They Are and Why They Look Brown#
Diatoms contain a pigment called fucoxanthin that gives them their characteristic brown or golden-brown color. They build intricate silica shells called frustules, and they reproduce explosively when conditions are right -- namely when dissolved silicates are available and competition from other photosynthetic organisms is low. In a new aquarium, both conditions are almost always met.
The brown film you see is millions of individual diatom cells. It wipes off easily with a finger or cloth, unlike green spot algae (which requires scraping) or black beard algae (which clings tenaciously). That easy-smear quality is your first diagnostic clue.
How to Tell Brown Algae Apart from Other Algae Types#
| Algae Type | Color | Texture | Where It Grows | Removal Difficulty |
|---|---|---|---|---|
| Brown algae (diatoms) | Brown, dusty | Slippery film, wipes off easily | Glass, gravel, decorations, plant leaves | Easy -- wipes or vacuums off |
| Green spot algae | Dark green spots | Hard, crusty | Glass, slow-growing plant leaves | Hard -- needs razor scraper |
| Black beard algae (BBA) | Dark grey-black tufts | Fuzzy, branching | Edges of leaves, filter outlets, driftwood | Difficult -- spot-treat with H2O2 or Excel |
| Cyanobacteria (blue-green) | Blue-green or dark red slime | Slimy sheet, peels off in mats | Substrate, rocks, low-flow areas | Moderate -- often needs blackout + antibiotics |
Brown algae is the easiest type to identify and remove.
Why Does Brown Algae Appear in Fish Tanks?#
Brown algae doesn't appear randomly. It's driven by a specific set of water chemistry conditions, and understanding those conditions tells you exactly how to fix the problem.
Excess Silicates -- the #1 Cause in New Tanks#
Diatoms need dissolved silica to build their cell walls. Most municipal tap water contains silicates (typically 5-25 ppm, varying by region according to USGS water quality data), and many substrates -- especially new gravel and sand -- leach additional silicates into the water column. In a new tank, silicate levels are at their peak because nothing has consumed or exported them yet.
Low Light Levels and the Light-Spectrum Problem#
Diatoms thrive under low and inadequate lighting because they can photosynthesize efficiently at light levels where green algae and most aquarium plants cannot compete. If you're running a stock hood light, a bulb past its useful life, or keeping lights on for less than 6 hours per day, you're giving diatoms a competitive advantage. Spectrum matters too -- diatoms do well under warm (yellowish) light, while full-spectrum bulbs closer to 6500K favor plants.
High Nitrates and Phosphates Feeding Diatom Growth#
Elevated nitrates (above 20 ppm) and phosphates (above 1 ppm) provide the nitrogen and phosphorus that all photosynthetic organisms need to grow. Overfeeding, overstocking, and infrequent water changes all contribute. If your established tank suddenly develops a diatom bloom, test nitrates and phosphates first -- they're almost certainly elevated.
New Tank Syndrome -- Why Brown Algae Is Almost Universal in Cycling Tanks#
In a cycling tank, nutrient levels are unstable, beneficial bacteria haven't fully established, and silicates from substrate and tap water are plentiful. This combination creates perfect diatom conditions. The bloom typically appears between weeks 2-6 and fades on its own as the tank matures. If your tank is less than two months old and you see brown algae, this is overwhelmingly the explanation.
A diatom bloom during cycling is a normal phase, not a failure. Nearly every new freshwater tank goes through it. Resist the urge to tear down the setup or dump in chemicals -- patience and basic maintenance are the correct response.
Is Brown Algae Harmful to Fish or Plants?#
Short-Term vs. Long-Term Impact on Water Quality#
Diatoms themselves are not toxic to fish or invertebrates. They don't produce harmful compounds, and many fish and snails eat them. In the short term, brown algae is purely a cosmetic problem. A thin film on the glass or a dusting on the gravel won't affect your fish at all.
Over the long term, heavy diatom buildup on plant leaves can block light and slow plant growth, but it takes a significant coating to cause real damage. The bigger concern is what the diatoms indicate about your water.
When Brown Algae Signals a Bigger Water Chemistry Problem#
If brown algae persists beyond 6-8 weeks in a new tank, or appears suddenly in an established tank, it's telling you that nutrient levels are out of balance. Persistent diatom blooms in mature tanks usually mean high silicates from tap water, elevated nitrates from overstocking or underfiltering, or both. The algae isn't the disease -- it's the symptom. Fix the chemistry and the diatoms disappear.
Manually removing diatoms without addressing the underlying cause is like mopping a floor while the faucet is running. You'll wipe the glass clean on Saturday and see brown film again by Tuesday. Test your water parameters, identify what's feeding the bloom, and fix that. The wiping stops when the chemistry is right.
How to Remove Brown Algae (Step-by-Step)#
Manual Removal -- Glass Scraper, Gravel Vacuum, Wiping Decorations#
Start with physical removal. Use an algae scraper or magnetic cleaner on the glass, a gravel vacuum on the substrate, and a soft brush or old toothbrush on decorations. For plant leaves, gently rub them between your fingers underwater. Remove heavily coated decorations and rinse them in old tank water (never tap water, which kills beneficial bacteria on porous surfaces).
Water Change Protocol -- How Much and How Often During a Bloom#
During an active diatom bloom, increase your water change frequency. Do 25-30% water changes twice weekly instead of once. This exports dissolved silicates, nitrates, and phosphates that fuel diatom growth. If your tap water is high in silicates (test it -- kits are available from most aquarium suppliers), consider switching to RO/DI water for water changes during the bloom.
Algae-Eating Fish and Invertebrates That Target Diatoms#
Biological control is your best long-term weapon against diatoms. Several species actively graze brown algae and will keep it in check once the worst of the bloom is manually removed.
| Species | Tank Size | Group Size | Effectiveness | Notes |
|---|---|---|---|---|
| Otocinclus catfish | 10+ gallons | 6+ (schooling) | Excellent | Best diatom grazer; needs mature tank with biofilm |
| Nerite snails | 5+ gallons | 1-2 per 5 gal | Excellent | Won't reproduce in freshwater; great for small tanks |
| Amano shrimp | 10+ gallons | 3-5 per 10 gal | Good | Also eat other algae types; need a lid (climbers) |
| Bristlenose pleco | 20+ gallons | 1 | Good | Eats diatoms when young; needs driftwood and space |
| Mystery snails | 5+ gallons | 1-2 | Moderate | Will eat diatoms but prefer other food sources |
Always verify compatibility with your existing fish before adding new species.
You can use the compatibility checker to confirm these species work with your current livestock before buying. For the best selection, visit a local aquatic store where staff can advise on stocking levels for your specific tank.
Don't add otos to a tank less than 3 months old. They're sensitive to water quality swings and need established biofilm to supplement their diet. In a brand-new cycling tank, they'll starve or die from ammonia exposure. Wait until the cycle is fully complete and the tank has some maturity.
Chemical and Commercial Treatments -- When to Use Them and What to Avoid#
Chemical algae treatments exist but are rarely necessary for diatoms. Phosphate-removing filter media (like Seachem PhosGuard) can help in tanks with chronically high phosphate levels. Silicate-absorbing resins are available but are a band-aid -- your maintenance routine and water source are the real levers.
Avoid copper-based algaecides entirely. They're toxic to invertebrates, can crash your biological filter, and are complete overkill for a diatom problem that will resolve with basic husbandry.
How to Prevent Brown Algae from Coming Back#
Upgrading or Adjusting Your Lighting (Spectrum, Duration, Intensity)#
Switch to a full-spectrum LED in the 6500K range if you haven't already. Run your lights 8-10 hours per day on a timer. This duration and spectrum favor plant growth over diatom growth. If you don't have live plants, reduce the photoperiod to 6-8 hours -- without plants to outcompete them, more light just feeds algae of all types.
Controlling Silicates -- RO/DI Water vs. Tap Water#
If your tap water tests high for silicates (above 10-15 ppm), switching to RO/DI water for water changes and top-offs is the most effective long-term prevention. RO/DI units remove 95-99% of dissolved silicates along with other impurities. You'll need to remineralize with a product like Seachem Equilibrium to restore essential minerals for fish and plants.
Nutrient Export: Live Plants, Regular Water Changes, Proper Feeding#
Live plants are your best ally against all types of algae. Fast-growing species like hornwort, water sprite, and floating plants outcompete diatoms for nutrients. Combine planted tanks with a consistent water change schedule (25-30% weekly) and careful feeding (only what fish consume in 2-3 minutes, once or twice daily) to keep nitrates and phosphates low.
Substrate Choice and Depth -- How Gravel Traps Silicates#
New substrates, particularly play sand and coarse gravel, leach silicates into the water column. Rinsing substrate thoroughly before use reduces this, but some leaching is inevitable. Deeper substrate beds trap more detritus and can become silicate reservoirs over time. A depth of 1-2 inches is sufficient for most setups; substrate depth affects silicate buildup, so avoid going deeper than necessary unless you're running a planted tank that requires it.
Brown Algae in Specific Tank Types#
New Tank (Cycling) -- What's Normal and When to Worry#
A diatom bloom during weeks 2-8 of a new tank is textbook normal. It appears, peaks, and fades as silicate levels drop and the biological filter matures. You should only be concerned if the bloom persists beyond 8-10 weeks, intensifies rather than fading, or is accompanied by ammonia or nitrite readings above zero. In those cases, something else is wrong -- typically an incomplete cycle, overstocking, or a persistent silicate source.
Planted Tanks -- Balancing Light and CO2 to Outcompete Diatoms#
In planted tanks, diatoms usually signal that the plant-to-nutrient ratio is off. Plants aren't growing fast enough to consume available nutrients, so diatoms fill the gap. The fix is to boost plant growth: ensure adequate lighting (6500K, 8-10 hours), consider adding CO2 injection if you're running a high-tech setup, and dose a complete fertilizer to prevent any single nutrient deficiency from limiting plant growth. Healthy, fast-growing plants will starve diatoms out within weeks.
Saltwater/Reef Tanks -- Diatom Blooms on Sand Beds#
Diatom blooms in new saltwater tanks are just as common as in freshwater. They typically coat the sand bed in a brown film within the first 2-4 weeks. The same fundamentals apply: excess silicates from new sand (especially live sand and aragonite) and unstable nutrient levels drive the bloom. Turbo snails, cerith snails, and nassarius snails are the cleanup crew of choice. Most reef keepers run RO/DI water from day one, which shortens the diatom phase considerably. If your reef tank has persistent diatoms, find a local reef specialist who can test your water source and advise on filtration upgrades.
Quick-Reference: Brown Algae Troubleshooting Checklist#
| Parameter | Target | Notes |
|---|---|---|
| Tank age | Under 8 weeks | Normal cycling diatoms -- will resolve on their own with basic maintenance |
| Silicates (SiO2) | Below 5 ppm | Test tap water; switch to RO/DI if above 10-15 ppm |
| Nitrates (NO3) | Below 20 ppm | Increase water changes if elevated; reduce feeding |
| Phosphates (PO4) | Below 1 ppm | Use phosphate-removing media if chronically high |
| Lighting | 6500K, 8-10 hrs/day | Low light favors diatoms; upgrade to full-spectrum LED |
| Water changes | 25-30% weekly | Twice weekly during active bloom |
| Algae crew | Otos, nerite snails | Add only after tank is cycled and at least 3 months old (for otos) |
Step 1: Identify your tank age. Under 8 weeks? This is normal cycling behavior -- maintain routine care and wait.
Step 2: Test water. Check silicates, nitrates, and phosphates. If any are elevated, you've found your cause.
Step 3: Manual removal. Scrape glass, vacuum gravel, wipe decorations. Do a 25-30% water change.
Step 4: Fix the source. Switch to RO/DI water if silicates are high. Increase water changes if nitrates/phosphates are high. Upgrade to 6500K full-spectrum lighting.
Step 5: Add biological control. Nerite snails for small tanks, otocinclus for tanks 10+ gallons (only if cycled 3+ months).
Step 6: Prevent recurrence. Maintain weekly water changes, don't overfeed, keep plants healthy, and run lights on a timer.
Timeline: Most blooms clear within 2-4 weeks of consistent action. New-tank diatoms resolve within 4-8 weeks naturally.
Where to Find Algae-Eating Fish and Supplies#
The best place to source otocinclus, nerite snails, and other diatom grazers is a local fish store where you can inspect the animals in person. Healthy otos should be actively grazing on surfaces with rounded bellies -- avoid any with sunken stomachs, which indicates starvation and is difficult to reverse. A good LFS will also carry silicate test kits, RO/DI water, and phosphate-removing filter media that big-box stores often don't stock.