Diatoms (Brown Algae) in Aquarium: Causes and How Long It Lasts

What diatoms actually are

Brown algae is a misleading name. What you're looking at isn't true algae in the botanical sense - it's diatoms, a group of photosynthetic microorganisms that build their cell walls from silica (silicon dioxide). Under a microscope they're genuinely beautiful: intricate glass boxes in thousands of geometric patterns. In your tank, they just look like someone dusted everything with cocoa powder overnight.

The film wipes off glass effortlessly with a finger or cloth. That's your first clue it's diatoms rather than green spot algae or cyanobacteria - nothing else comes off quite so easily or reappears quite so quickly. Leave it and the coating becomes denser over days, sometimes taking on a slightly slimy texture as the colony thickens.

Diatoms are almost universal in new freshwater setups. If your tank is under three months old and you're seeing this, you're not doing anything wrong. You're watching a completely normal chemical process play out.

Why new tanks get diatoms - the silica cycle explained

Diatoms need dissolved silica (orthosilicic acid, Si(OH)₄) to build their shells. New aquarium substrates - especially coarse gravels, sand, and many planted tank soils - leach silicates as they settle and break down. Tap water in most regions also carries measurable silicates, typically 10-40 ppm, sometimes higher depending on your local geology and water treatment process.

In a new tank, there's no competing biofilm to outcompete the diatoms for light and nutrients. The bacterial colony that will eventually cover every surface is still establishing itself as part of the nitrogen cycle. Diatoms move into that vacancy fast - they're among the first colonisers of any new wet surface.

As the tank matures, two things happen simultaneously. First, the silicate reservoir in your substrate depletes - it's not infinite. Second, a diverse microbial biofilm develops on every surface, reducing the available silicate further and physically crowding out diatom colonies. The result is that diatoms fade on their own, even if you do nothing at all.

Is your tap water making it worse?

Possibly, yes. If your diatoms are unusually heavy or persist past the normal window, your tap water silicate level is worth testing. You can buy a silicate test kit from any decent aquarium retailer for a few pounds or dollars. Readings above 30-40 ppm from the tap will continuously replenish what the substrate would otherwise exhaust, extending your diatom phase significantly.

The fix in that case isn't a bottle of chemicals - it's physical: a small reverse osmosis (RO) unit removes silicates from source water almost completely. RO water does strip everything else too, so you'd remineralise it to your target water parameters before use. That's a more involved solution most beginners don't need. If your tap silicates are moderate and your tank is simply new, you'll outgrow the problem without intervention.

Light and diatoms: the relationship most guides miss

Diatoms are low-light specialists. They evolved in deep or turbid water where they outcompete green algae that need stronger light. In aquariums, long photoperiods with weak or diffuse light - a common setup for new tanks where people are being cautious - actually favour diatom growth over the green algae and plants that would eventually displace them.

This creates a counterintuitive situation: running your lights for 10-12 hours at low intensity gives diatoms a competitive advantage. A tighter photoperiod of 6-8 hours at appropriate intensity for your plant load is better. If you're running a low-tech planted tank, 7 hours at moderate intensity (around 20-40 PAR at substrate level) tends to suppress diatoms faster than a dim 12-hour day.

The same logic applies to tanks placed near windows. Indirect natural light, especially morning light that's bright but not direct, is enough to sustain diatom colonies far longer than you'd expect from a "dim" setup.

Algae eaters that actually help - and how to use them correctly

Diatoms are nutritious and soft, which makes them ideal food for grazing invertebrates and certain fish. The two species most effective in home aquariums are nerite snails and Otocinclus catfish.

Nerite snails (Neritina spp.)

The most reliable glass-cleaners in the hobby. A single nerite will methodically work its way around a 60-litre tank, scraping diatoms off every surface it can reach. They're safe with all peaceful fish, won't breed in freshwater (eggs are deposited but don't hatch without brackish conditions), and tolerate a wide parameter range. For diatom control, stock 1 nerite per 15-20 litres as a rough guide. Zebra nerites (Neritina natalensis) and olive nerites work equally well - the choice is aesthetic.

Otocinclus catfish (Otocinclus spp.)

Excellent at grazing diatoms from plant leaves and hardscape, but they have a critical requirement most beginners underestimate: they need an established tank with a mature biofilm before you add them. Otocinclus are wild-caught (most aren't bred commercially), arrive stressed and thin, and need constant access to biofilm and algae to survive. Add them to a tank mid-cycle or before week four and you'll find them dead within days.

Wait until your tank has fully cycled - zero ammonia, zero nitrite - and has visible algae growth on surfaces before purchasing Otocinclus. Buy from a supplier where the fish have been held for at least a week and are actively feeding. Stock in groups of 6 or more; they're social and fare poorly alone. A group of 6 in a 75-litre tank will make a visible difference within 48 hours.

Mechanical removal: what's worth doing

Wiping diatoms off glass with a magnetic cleaner or cloth does help, but it's temporary - the colony regrows from cells that remain on the surface and from water column diatoms settling again. That said, regular removal reduces the biomass competing for silicates, slightly accelerating the depletion process. It also keeps your tank looking presentable while you wait out the cycle.

For plants, a soft brush or even your fingers works fine - diatom film comes off without damaging leaves. Don't bother trying to clean porous hardscape like lava rock or rough driftwood; the texture traps diatom colonies in crevices that grazers actually handle better than manual cleaning does.

Water changes help too, but not in the way most people think. A water change dilutes dissolved silicates temporarily, but if your tap water contains silicates (which most does), you're partially replenishing what you just diluted. The benefit is still real - dilution plus removing diatom-laden water column particles - but don't expect a large water change to solve the problem overnight. Consistent regular water changes at 20-30% weekly during the diatom phase are more effective than infrequent large ones.

Diatoms in established tanks: when it's not just "new tank syndrome"

If your tank is more than three or four months old and diatoms suddenly appear or worsen, something has changed. Common triggers in established aquariums include:

• New substrate added: Even topping up sand or adding a new bag of gravel introduces fresh silicates. Expect a brief diatom pulse lasting 2-4 weeks.
• New driftwood or rocks: Some silica-rich stones (particularly certain granites and volcanic rocks) leach silicates continuously. If diatoms appear specifically on and around new hardscape, the material is the source.
• Reduced grazing pressure: If your nerites or Otocinclus have died without replacement, diatom populations rebound quickly.
• Lighting change: Switching to a dimmer bulb, moving the tank to lower ambient light, or reducing photoperiod below 6 hours can tip the balance back toward diatoms.
• Tap water silicates: Some municipal water sources see seasonal variation. If diatoms recur every spring or after heavy rains, your supply water is worth re-testing.

Diatoms in an established tank are a diagnostic flag, not a catastrophe. Run the logic: what changed four to six weeks ago? The answer almost always points to the cause.

What diatoms won't do (reassurance where it's due)

Diatoms don't harm fish directly. They don't lower oxygen levels significantly, don't produce toxins, and don't damage plant leaves the way some algae can. Fish swim through diatom blooms without apparent distress. Plants may grow slightly more slowly if leaves are heavily coated - light penetration is reduced - but it's not the leaf damage you'd see from black beard algae or staghorn.

The main harm from diatoms is aesthetic and psychological: a new tank owner sees brown coating everywhere and assumes they've failed. You haven't. This is the tank working through a predictable chemical phase. If your fish are behaving normally and your parameters are in range, diatoms are the least of your concerns.

Summary: what to actually do

If your tank is under eight weeks old: wipe the glass weekly, run a 6-8 hour photoperiod, do consistent 20-25% water changes, and add 1-2 nerite snails if you want active help. Don't add Otocinclus until the cycle is complete and algae is well established. Don't buy bottled treatments. The diatoms will go.

If your tank is established and diatoms have returned: identify what changed, test silicates if the source isn't obvious, check that your grazing population is healthy, and review your light schedule. If you're adding new fish or invertebrates for grazing purposes, run them through the stocking calculator first to make sure your bioload stays manageable.