Diatoms in a Reef Tank: Why the Brown Dust Shows Up and How to Handle It

If your new reef tank has developed a dusty brown coating on the sand, glass, and lower rockwork a couple of weeks after setup, you haven't done anything wrong — you're looking at a diatom bloom, one of the most predictable and least concerning phases of a new tank's first few months. It's not harmful, it doesn't need a treatment product, and in almost every case it resolves on its own.

Short Answer: What's Happening and What to Do

That brown film is a diatom bloom — a population explosion of single-celled algae that build their cell walls from silica. It's most common in tanks 1-4 weeks old, fueled by the silicates naturally present in new sand, dry rock, and source water. It's not harmful to anything in your tank, and it typically clears up within days to a few weeks as silicate levels drop and other organisms establish themselves. The best response is patience plus gentle removal during normal maintenance — not a special treatment.

What Diatoms Are and Why They Show Up

Diatoms are microscopic, single-celled algae found in nearly every body of water on Earth, both fresh and salt. What sets them apart from other algae is their cell wall, called a frustule, which is built from silica — the same element (in different forms) found in sand and glass. Where silica is abundant and other competing organisms haven't yet established themselves, diatoms can multiply explosively, coating surfaces in a brown, dusty film that often looks worse than it is.

A brand-new reef tank is, almost by definition, a silica-rich environment:

• New sand — especially aragonite or silica-based sand products — can leach silicates for the first several weeks after being added to a tank
• Dry rock — porous rock that hasn't been fully "soaked" can release trace silicates as it cures in tank water
• Source water — tap water commonly carries dissolved silicates, and even RO/DI water isn't guaranteed to be silicate-free if the system isn't fully dialed in (see acceptable TDS for reef tank source water for how to confirm this)
• Lack of competition — in a mature tank, diatoms have to compete with established macroalgae, coralline algae, bacteria, and a working cleanup crew for the same resources. A brand-new tank has none of that competition yet, so whatever diatoms are present have the field to themselves (our reef plants and algae overview covers what these competing algae types actually are and why most of what looks like "plants" on a reef is algae too)

This combination — abundant silica, minimal competition — is why diatom blooms are so closely associated with new tanks specifically, and why they're often described as part of the predictable "ugly stage" that follows the initial nitrogen cycle.

How Long the Bloom Lasts

For most tanks, a diatom bloom runs its course within a few days to a few weeks. The bloom typically follows a recognizable arc: it appears gradually, peaks (often looking its worst around 2-3 weeks into a new tank's life), and then fades as quickly as it arrived once the available silicate supply starts running out and competing organisms catch up.

If a brown film is still going strong two months or more after setup, that's no longer "new tank diatoms" running their natural course — it usually means something is continuing to supply silicates. The most common ongoing source at that point is the RO/DI water used for water changes and top-off. A system with exhausted DI resin can pass silicates straight through, and because top-off water is added so frequently, even a small ongoing silicate supply can sustain a diatom population indefinitely. Checking your RO/DI output with a TDS meter is the right next step if diatoms are overstaying their welcome — see our guide on acceptable TDS for reef tank source water for what to look for.

Are Diatoms Harmful?

No. This is the most important thing to know about a diatom bloom: it's a cosmetic issue, not a water quality or health issue. Diatoms aren't toxic to fish, corals, or invertebrates, and their presence doesn't indicate a cycling problem, an ammonia spike, or anything else that needs urgent attention.

The main downsides are purely visual:

• A film over the sand and lower glass that makes the tank look dirty
• A dusty coating on rockwork and equipment that can dull the appearance of corals or frags placed low in the tank
• Occasionally, diatoms can settle on coral tissue itself, though this is more of a "looks a bit dusty" issue than anything that harms the coral

It's worth saying directly: a diatom bloom is actually a sign that your tank is progressing normally through its early biological stages, not a sign of a problem to fix. Tanks that never show any diatom bloom at all aren't necessarily "better" — they may simply have started with lower-silicate materials.

How to Manage and Reduce Diatoms

While you don't strictly need to do anything, a few simple habits make the bloom less visually unpleasant and can shorten how long it sticks around:

1. Siphon gently during water changes. Use your water-change siphon to lightly vacuum the diatom film off the sand surface and glass. Avoid digging into the sand bed itself — stirring up the substrate can release trapped nutrients and prolong the situation rather than help it.
2. Wipe the glass. A magnetic algae scraper or pad removes diatom film from the viewing panes quickly, and unlike the sand, glass can be wiped as often as you like without consequence.
3. Let your cleanup crew work. Snails like Trochus and Cerith species graze on diatom films as part of their normal diet, and a tank that's stocked with an appropriate cleanup crew (introduced once ammonia and nitrite have cleared) will generally show diatoms clearing faster.
4. Maintain normal flow and lighting. There's no need to change your lighting schedule or increase flow specifically to "fight" diatoms — normal tank operation is part of what allows competing organisms to establish and eventually outcompete them.
5. Be patient before changing anything major. The most common mistake is overreacting to a normal diatom bloom by changing lighting schedules, adding chemical treatments, or doing unusually large water changes — none of which address the actual cause (silica availability) and all of which can introduce new instability to a tank that's still finding its balance. As your broader saltwater system matures, diatoms are one of several early-stage cosmetic issues — alongside cycling and initial parameter swings — that tend to resolve simply by giving the tank time.
6. If it persists past 6-8 weeks, check your source water. Test your RO/DI output's TDS. A reading above 0 ppm points to silicates (among other things) passing through, which is the most likely explanation for an unusually long-lasting bloom.

Quick Reference

• Brown, dusty film on sand/glass in a tank under ~2 months old is almost always a normal diatom bloom
• Diatoms are silica-shelled algae, fueled by silicates from new sand, dry rock, and source water
• Not harmful to fish, corals, or invertebrates — purely cosmetic
• Typically resolves within days to a few weeks on its own
• Siphon the sand surface and wipe glass gently during normal maintenance
• Let an appropriate cleanup crew (Trochus, Cerith snails) graze on the film
• Don't overreact with lighting changes or chemical treatments
• If diatoms persist past 6-8 weeks, test your RO/DI water's TDS for an ongoing silicate source