Why Your Aquarium Has Brown Diatoms (And the Fastest Ways to Fix It)
Brown dust on your glass? Plants turning rusty? Relax — it’s just diatoms, and you can fix them fast.
Brown diatoms are one of the most common early-stage aquarium headaches. Almost every new aquarist runs into them, and even experienced hobbyists see them return when tank conditions shift. The good news? Diatoms aren’t dangerous. They’re actually a sign your tank is still “settling in.”
Let’s break down why they appear and how to get rid of them quickly without stressing your fish or plants.
What Exactly Are Brown Diatoms?
Diatoms are a type of algae that appear as a thin, brown, dusty film on:
-
Glass
-
Substrate
-
Plant leaves
-
Filters
-
Decorations
Unlike green algae, brown diatoms wipe off easily with your finger — almost like loose dirt.
They explode in new tanks because they love silicates, low light, and unstable water conditions. Think of them as nature’s “first responders” working to stabilize your aquarium.
Why Your Tank Has Brown Diatoms
1. Your Aquarium Is New (0–3 Months Old)
New tanks go through the nitrogen cycle, and diatoms thrive in this unstable phase.
If your tank is still young, diatoms are completely normal. They usually disappear on their own once the cycle stabilizes.
2. High Silicates in Water or Substrate
Tap water, sand, and certain rocks naturally leach silicates — diatoms’ favorite food source.
If you’re using play sand or a fresh substrate, expect an early diatom bloom.
3. Weak Lighting or Short Photoperiods
Diatoms love low light.
If your lighting is weak, old, or only on for a few hours per day, they take over before your plants can.
4. Low Plant Mass (Not Enough Plants Competing for Nutrients)
Live plants outcompete algae. But in lightly planted tanks, diatoms have the upper hand.
The fewer the plants, the more aggressively diatoms spread.
5. Poor Plant Growth or Nutrient Imbalance
Underfed or struggling plants can’t fight off algae.
When plants aren’t growing well, diatoms move in and fill the gaps.
How to Get Rid of Brown Diatoms FAST
Here are the most effective steps to clear your tank in days — not weeks.
1. Add More Live Plants (Especially Fast-Growing Ones)
Increasing plant mass is the most reliable long-term fix.
Fast growers help stabilize the tank and starve out diatoms.
2. Increase Your Lighting (8 Hours a Day)
Set your aquarium light to:
-
Around 8 hours daily
-
Medium brightness
Too little light gives diatoms the advantage.
3. Improve Water Flow
Diatoms thrive in stagnant water.
A sponge filter or improved circulation helps prevent buildup and supports plant health.
4. Wipe Surfaces and Siphon Them Out
Diatoms are incredibly easy to remove:
-
Wipe the glass
-
Shake plants gently
-
Siphon debris during water changes
This instantly removes a huge amount of the problem.
5. Let Your Tank Mature
Even without intervention, diatoms usually disappear once the tank reaches stability.
Age is one of the strongest natural cures.
6. Add Algae-Eating Helpers (Optional)
These species help clean diatoms:
-
Nerite snails
-
Ramshorn snails
-
Otocinclus catfish
-
Amano shrimp
They won’t solve the root cause but are great cleanup crews.
How Long Until Diatoms Go Away?
Most tanks clear up within:
-
7–14 days with basic cleanup
-
3–8 weeks if left alone
-
Faster with strong plant growth and stable lighting
Once the tank stabilizes, diatoms stop finding a foothold.
Final Thoughts
Brown diatoms look ugly, but they’re harmless and very temporary.
They’re mostly a sign of a new or unbalanced aquarium — not a problem you caused.
By:
-
Adding more plants
-
Improving lighting
-
Increasing flow
-
Wiping away buildup
-
Allowing the tank to mature
…you can get rid of them faster than you might expect.
🌿 Want a Hardy Plant That Helps Beat Diatoms Naturally?
Amazon Swords are one of the best plants for stabilizing new aquariums and outcompeting algae.