What diatoms are and why they spread

Diatoms are single-celled algae with hard, glass-like shells made of silica. They appear as a brown or tan film on glass, substrate, plants, and decorations — not the green slime of other algae. Diatoms thrive in new tanks and in tanks with high silica levels, which come from tap water, sand, and some types of rock.

A new aquarium almost always gets diatoms in the first four to eight weeks because the tank is still establishing its bacterial balance and nutrient cycle. Diatoms feed on silica and excess nutrients, so they bloom when both are present. They are not toxic to fish, but they cloud the tank and make it look neglected.

The good news: diatoms die off on their own once the tank matures and silica runs low. You can speed that process by removing silica from the water and reducing the nutrients diatoms feed on.

Key Takeaways

  • Diatoms are brown algae that appear in new tanks because of high silica levels and excess nutrients; they fade naturally as the tank matures.
  • Using reverse osmosis water or a silica-removing resin cuts the main food source diatoms need to grow.
  • Increasing water changes and reducing light hours slows diatom growth while the tank establishes itself.
  • Adding diatom-eating fish like plecos or otocinclus can help, but they work best alongside other methods, not alone.
  • Scrubbing the glass and siphoning the substrate removes diatoms faster than waiting for them to disappear naturally.

Use reverse osmosis water or silica-removing resin

The fastest way to starve diatoms is to remove silica from your water. Tap water contains silica in varying amounts depending on your location — some areas have very little, others have enough to feed diatoms for weeks. Reverse osmosis (RO) water strips out silica along with other minerals and dissolved solids.

You can buy RO water at fish stores or grocery stores (often sold as distilled water for irons and humidifiers), or you can run your tap water through an RO filter at home. A small RO unit costs between $100 and $300 and produces water slowly, so it works best if you plan ahead. Mix RO water with tap water at a ratio that keeps your tank's mineral content stable — usually 50/50 or 75/25 RO to tap, depending on your tap water hardness.

If RO water is not practical, use a silica-removing resin in your filter. These resins bind silica molecules and pull them from the water as it flows through. They cost $20 to $40 per cartridge and last two to four weeks before needing replacement. Brands like Purigen and some specialty aquarium resins work this way, though you will need to check the product label to confirm it targets silica specifically.

Increase water changes and reduce light hours

Diatoms feed on excess nutrients — mainly nitrogen and phosphorus — that build up in new tanks before the nitrogen cycle is fully established. Larger, more frequent water changes remove these nutrients faster than the bacteria can consume them. Change 25 to 50 percent of the water every two to three days for the first month, then drop to weekly changes once the tank is established.

Reducing light also slows diatom growth. Diatoms photosynthesize, so they grow faster under bright light for long hours. Cut your light to six to eight hours per day instead of the typical ten to twelve. This does not harm most fish, and it gives the tank's bacteria time to catch up to nutrient levels. Once the brown film stops spreading, you can return to normal lighting.

These two steps work together: more water changes remove the nutrients, and less light reduces the diatoms' ability to use them. Neither method eliminates diatoms alone, but combined they speed up the tank's transition from new to mature.

Add diatom-eating fish and invertebrates

Several fish and shrimp eat diatoms directly. Otocinclus catfish are small (one to two inches) and graze constantly on algae films, including diatoms. They work best in groups of three or more and need calm tank mates and plenty of hiding spots. Plecos also eat diatoms, but most species grow large (six inches or more) and produce a lot of waste, which can make the diatom problem worse in a small tank.

Amano shrimp and ghost shrimp eat diatoms and other algae, though they are slower than fish and work best in tanks without aggressive fish. They also need stable water conditions and will not survive in a tank that is still cycling.

Do not rely on these animals alone. They help, but they cannot outpace diatom growth if silica and nutrients are still high. Add them after you have started removing silica and increasing water changes. They work best as part of a full approach, not as a single solution.

Scrub the glass and siphon the substrate regularly

Manual removal speeds up the process. Use a soft algae scraper or a clean cloth to wipe diatoms off the glass two or three times per week. This does not kill the diatoms permanently — they will return — but it keeps the tank looking clear while the underlying conditions improve.

Siphon the substrate during water changes to remove diatom film from the sand or gravel. Diatoms settle on the bottom and in crevices, and vacuuming pulls them out along with decaying food and waste. This also removes excess nutrients that feed the diatoms, so it serves double duty.

Do not use chemical algae killers or hydrogen peroxide treatments on diatoms. These can harm fish and plants, and they do not address the root cause — high silica or excess nutrients. Once you fix those, diatoms disappear without chemicals.

Wait for the tank to mature

The most reliable solution is time. As the nitrogen cycle establishes and bacteria populations stabilize, nutrient levels drop and diatoms starve. Most tanks stop producing diatoms after six to twelve weeks, even without intervention. The brown film fades, and you are left with a clear tank.

If you do nothing but perform regular water changes and scrape the glass, diatoms will still go away — it just takes longer. The methods above speed up the process, but none of them are required. New tank diatoms are normal and temporary.

Once diatoms are gone, they rarely return unless you add new silica-rich substrate or your tap water silica levels spike. If they come back months later, check whether you have changed your water source or added new sand or rock.

Frequently Asked Questions

Are diatoms harmful to fish?

No. Diatoms do not produce toxins and do not consume oxygen. They are unsightly and can block light from reaching plants, but fish tolerate them without issue. They are a cosmetic problem, not a health problem.

Can I use distilled water instead of reverse osmosis water?

Yes, distilled water has silica removed, just like RO water. It is often cheaper at grocery stores. Mix it with tap water the same way you would RO water to avoid mineral deficiencies in the tank.

Will a UV sterilizer kill diatoms?

No. UV sterilizers kill free-floating algae spores in the water column, but diatoms live on surfaces — glass, substrate, plants — where UV light does not reach them. A sterilizer will not reduce the brown film you see.

Do I need to remove all the substrate and start over?

No. Rinsing the substrate during water changes removes diatom film without replacing it. If your substrate is silica-rich sand, you can replace it with inert sand or gravel, but this is only necessary if diatoms persist after twelve weeks and other methods have not worked.

What if diatoms come back after the tank is mature?

Check your tap water silica levels or whether you have added new substrate recently. If neither has changed, the return usually means nutrient levels have risen — often from overfeeding or a dead spot in the substrate. Increase water changes and reduce feeding, then monitor for two weeks.