What diatoms are and why they appear

Diatoms are single-celled algae with hard, glass-like shells made of silica. In a saltwater aquarium, they show up as a thin brown or tan film on glass, rocks, sand, and decorations. They are not harmful to fish, but they cloud the view into your tank and can cover surfaces faster than you'd expect.

Diatoms thrive in new aquariums or after water changes because they feed on silica and excess nutrients in the water column. They are especially common in the first four to eight weeks of a tank's life, when the biological balance is still settling. High silica levels—often from tap water or new substrate—speed their growth. Poor lighting, low water flow, and infrequent water changes also create conditions where diatoms spread unchecked.

The good news is that diatoms are temporary. Once your tank matures and silica depletes, they usually vanish on their own. Until then, a combination of physical removal and water management will keep them under control.

Key Takeaways

  • Diatoms are brown algae that feed on silica and excess nutrients; they are harmless but unsightly and usually disappear as your tank matures.
  • Perform 25 percent water changes twice a week to lower silica and nutrient levels that fuel diatom growth.
  • Use a soft brush or cloth to wipe diatoms off glass and rocks by hand, since they do not respond well to chemical treatments.
  • Increase water flow with a powerhead or adjust your return pump to help prevent diatoms from settling on surfaces.
  • Introduce diatom-eating animals like sea urchins, starfish, or certain snails to reduce the population naturally over time.

Water changes and silica reduction

The fastest way to starve diatoms is to remove the silica and excess nutrients they feed on. Perform a 25 percent water change twice a week for the first month, then drop to once a week as the diatom bloom fades. This removes dissolved silica from your tap water and waste products that accumulate between changes.

If your tap water is high in silica, consider using reverse osmosis (RO) water instead. RO water strips out silica and other minerals, which means you will need to add back the minerals your fish and corals need—but it stops diatoms at the source. Many aquarists mix RO water with tap water to balance cost and silica reduction. You can buy RO water at most aquarium stores or fish shops by the gallon, or invest in an RO unit for your home if you plan to keep saltwater tanks long-term.

Do not skip water changes hoping diatoms will leave faster on their own. Stagnant water with high nutrient levels only extends the bloom. Consistent, frequent changes are the single most effective control.

Physical removal with brushes and cloths

Diatoms wipe away easily by hand because they have no strong hold on surfaces. Use a soft-bristled brush or a clean cloth to scrub the inside of the glass, rocks, and decorations. A standard aquarium algae brush works well, or you can use a soft toothbrush for tight spaces. Avoid steel wool or abrasive pads, which scratch acrylic tanks and can damage delicate corals if you brush near them.

Wipe the glass during or just before a water change so the diatoms you remove stay out of the tank. If you brush during the change, the diatoms will settle back down once the water is still. Some aquarists use a magnetic algae scraper on the outside of the glass to pull diatoms off without entering the tank, which is faster if your tank is large.

Do not rely on chemical algae treatments for diatoms. Most algaecides do not target diatoms effectively and can harm fish or corals. Physical removal combined with water management is safer and more reliable.

Increasing water flow and circulation

Diatoms settle and stick to surfaces in low-flow areas. Increasing water movement makes it harder for them to attach and gives them less time to establish. If your tank has dead spots—corners or behind decorations where water barely moves—add a small powerhead or adjust your return pump to direct flow into those areas.

Aim for moderate, steady flow rather than strong turbulence. Most fish and corals prefer gentle, consistent movement. A powerhead set to oscillate (turn on and off) or rotate can help without creating stress. Even a small circulation pump running 4 to 6 hours a day in a stagnant corner will reduce diatom buildup there.

Better flow also improves oxygen levels and nutrient distribution, which benefits your whole tank. It is a change that helps beyond just diatom control.

Animals that eat diatoms

Several saltwater animals feed on diatoms and can reduce the population naturally. Sea urchins (especially the red or blue varieties) are aggressive diatom eaters and will graze on rocks and glass throughout the day. Starfish like the red fromia or brittle star also consume diatoms, though they move more slowly. Certain snails—particularly the turbo snail and astrea snail—are effective diatom grazers, though they may also eat coralline algae (the purple or pink algae you usually want to keep).

Add these animals only if your tank is stable and you have room for them long-term. A sea urchin needs space to roam and will not survive in a tiny tank. Snails can overpopulate if food is abundant, so monitor their numbers. These animals work best as part of a broader strategy—they slow diatom growth but do not eliminate it on their own if silica levels remain high.

Do not add animals solely to fight diatoms if you are not prepared to keep them. Once the diatom bloom ends, you will have a permanent resident that may not fit your tank's long-term plan.

Lighting and nutrient balance

Diatoms thrive under any light, but excessive light duration can speed their growth. If your lights are on more than 10 to 12 hours a day, reduce the photoperiod by an hour or two. This slows diatom reproduction without harming most fish or corals, which adapt quickly to slightly shorter days.

Excess nutrients from overfeeding or decaying food also fuel diatoms. Feed only what your fish will eat in two to three minutes, once or twice a day. Remove uneaten food after a few minutes with a net. Overfeeding is one of the most common mistakes in new tanks and directly leads to longer diatom blooms.

If you have a protein skimmer, make sure it is running properly and adjusted to produce a wet, dark skimate (the waste it removes). A skimmer removes excess nutrients before they break down into silica and other compounds that diatoms feed on.

Timeline and when diatoms disappear on their own

In most saltwater aquariums, diatoms peak around week two to four and begin to fade by week six to eight. This timeline assumes you are doing regular water changes and maintaining basic tank care. If you do nothing, diatoms can persist for months because silica and nutrients remain available.

The bloom ends when silica is depleted and your tank's biological balance stabilizes. Beneficial bacteria colonize the substrate and filter, consuming excess nutrients before they accumulate. Coralline algae (the purple or pink algae on rocks) also begins to grow and outcompetes diatoms for space and resources.

If diatoms are still heavy after eight weeks despite regular water changes, check your tap water for high silica content or look for hidden sources of excess nutrients—a dead spot in the substrate, a clogged filter, or overfeeding. Addressing the root cause will speed recovery.

Frequently Asked Questions

Are diatoms dangerous to fish or corals?

No. Diatoms do not produce toxins and do not harm fish or corals directly. They are a nuisance because they cloud the view and can cover surfaces, but they will not kill anything in your tank. They are a sign of a new or imbalanced system, not a disease.

Can I use an algae scraper on live rock without damaging it?

Yes, but be gentle. Use a soft brush or cloth on delicate corals or coralline algae. A magnetic scraper or hard brush is safe on bare rock and glass. Avoid scraping areas where corals are attached, as you can break their base or cause stress.

Will adding more fish speed up diatom removal?

No. More fish produce more waste, which increases nutrients and prolongs the diatom bloom. Wait until diatoms fade before adding fish beyond your original stocking plan. Focus on water changes and physical removal instead.

Do diatoms mean my tank is dirty?

Not necessarily. Diatoms are normal in new tanks and do not indicate poor care. They are a natural part of the cycling process. However, if they persist beyond eight weeks, it usually means water changes are infrequent, silica levels are high, or nutrients are accumulating—all fixable with the steps in this guide.

Can I use RO water right away, or do I need to wait?

You can use RO water when ready. Mix it with tap water to reduce cost, or use it straight if you add a mineral supplement for fish and corals. RO water has no silica, so it will not fuel diatom growth. Start using it as soon as your tank is set up if silica is a problem.