Brown Algae on Your Sand? Diatoms in a New Tank, Explained

Somewhere around week two to six, every new saltwater tank does the same thing: the pristine white sand develops a rust-brown film, the rocks get dusted in what looks like cinnamon, and the glass grows a tea-colored haze. New keepers reliably assume they've failed. You haven't. You've hit the diatom bloom — the most universal, most predictable, and most harmless phase in all of reefkeeping.

Here's what diatoms actually are, why literally every new tank grows them, how long they last, and the short list of things worth doing (plus the longer list of things to not do).

What diatoms are

Diatoms are single-celled algae that build their cell walls out of silica — glass, essentially. That one biological quirk explains everything about the bloom:

• New tanks are full of dissolved silicates. They leach from fresh sand, new rock, and — if you use it — tap water. Silica is diatom fertilizer, and a new tank is a silica buffet.
• Diatoms are ultra-fast colonizers, the weeds that sprout first on any disturbed ground. They arrive before their competitors (green algae, coralline) and before their predators (your future cleanup crew) exist in any numbers.
• When the silica runs out, they crash. Unlike most algae problems, diatoms consume a resource your tank stops supplying — so the bloom is self-terminating by design.

You can identify diatoms by look and touch: a rust-brown to golden-brown dusty film, thin and uniform, that coats sand, rock, and glass. Wipe it and it comes off in a smoky puff rather than in strands. If it wipes away effortlessly and looks like brown dust, it's diatoms. (Stringy brown mats with trapped bubbles are dinoflagellates — a different, meaner beast, and rare in brand-new tanks. Green fuzz is hair algae, a later chapter.)

Why your tank has them (yes, yours specifically)

The diatom bloom typically starts two to six weeks after water hits the tank — right around the end of the cycle — and it happens to $10,000 builds exactly as it happens to starter kits. Fresh aragonite sand leaches silicate. Dry rock leaches silicate. New tanks have zero competition and zero grazers. It is not a water-quality failure; it's ecological succession, the same reason weeds precede grass on bare soil.

The one version that is self-inflicted: tanks topped off or mixed with tap water, which continuously resupplies silicate (and phosphate) and can keep a "two-week phase" running for months. If you're on tap water, the diatom bloom isn't a phase — it's a subscription.

How long it lasts

Two to eight weeks, typically. Small tanks with modest silicate reserves clear in two or three; tanks with silicate-rich rock or sand can run six to eight. It often ends as abruptly as it began — one week the sand is brown, the next it's white and the film doesn't return. What replaces it is the next stage of succession: green film algae, then coralline. Each phase crowds out the last.

What to actually do (a short list)

1. Wait. Genuinely the core strategy. The bloom burns out its fuel supply on its own.
2. Use RO/DI water, always. For mixing salt and for top-off. This is the single real lever: it caps the silicate supply at what the hardware leaches, which is finite. A 4-stage RO/DI unit (or purchased RO water) turns the bloom from open-ended into strictly temporary.
3. Keep the glass clean for your own sanity. A magnetic algae scraper makes it a 60-second daily habit. Cleaning the glass doesn't shorten the bloom — it just keeps the tank enjoyable while nature works.
4. Stir the sand surface lightly during weekly maintenance and siphon what lifts off. Cosmetic, satisfying, harmless.
5. Start your cleanup crew as the bloom fades. Trochus and cerith snails graze diatoms enthusiastically — a modest crew added near the end of the cycle mops the tail of the bloom. Sizing and species are covered in our cleanup crew stocking guide.

What NOT to do (the important list)

• Don't nuke it with chemicals. Silicate absorbers and "algae fix" bottles are solutions to a problem that solves itself, and they cost you money while teaching you nothing.
• Don't do panic water changes. Normal weekly changes, fine. Massive changes to "remove the brown" just wash the tank while the silicate keeps leaching — and if your change water is tap, you're refueling the bloom.
• Don't cut the lights to zero. Diatoms run on silica more than light; blackouts barely touch them but do stress photosynthetic livestock. Normal photoperiods are fine.
• Don't rehome the tank's plan. New keepers see brown sand and start buying gear, dosing bottles, and rethinking everything. The tank isn't broken. It's maturing on schedule.
• Don't confuse it with a cycle problem. Diatoms say nothing about ammonia. If you want reassurance, verify the cycle finished properly — our guide to cycle timelines includes the 24-hour test that settles it.

The bigger picture: the uglies

Diatoms are chapter one of what reefkeepers call the uglies — the months-one-through-four parade of brown film, green film, and often some hair algae that every tank walks through while its ecosystem finds equilibrium. Tanks that get through the uglies gracefully all share one trait: a keeper who kept feeding lightly, kept up small water changes, used pure water, and didn't overreact. Panic moves — chemical warfare, daily rescapes, blackouts, gear-buying sprees — reliably extend the ugly phase they were meant to shorten.

The full tour of the uglies, phase by phase with timelines, is a chapter of our beginner's guide, The First Tank. Read it once and the brown sand becomes what it actually is: a progress bar.

FAQ

Are diatoms harmful to fish or corals?

No. The film is unsightly, not toxic, and it doesn't smother healthy corals at normal bloom thickness. The only real caution: a very heavy film on the sand can mat over and trap detritus, so give the surface a light stir at maintenance time.

My diatoms came back after months. What does that mean?

A silicate source re-entered the system. The usual suspects: a switch (or slip) to tap water, a new bag of sand, new rock, or exhausted DI resin letting silicates through your RO/DI unit. Check your top-off water first and test your RO/DI's TDS — a reading above zero means the resin needs replacing.

Will a cleanup crew prevent the diatom bloom entirely?

Prevent, no — snails can't out-eat a bloom running on a fresh silicate reserve, and stocking a full crew before the tank grows food risks starving them. The right sequence: let the bloom start, add a modest crew as the cycle completes, and let the grazers meet the diatoms in the middle.