How to Get Rid of Brown Algae (Diatoms) in a New Tank

You set the tank up a week or two ago. The water finally cleared, the stones look great, and then one morning a dusty brown film is on the glass, the rocks and the driftwood. That film is diatoms, and in a new tank it's normal. Wipe it off, siphon it out and give it time. It usually fades on its own as the tank settles. Below I cover what it is, what feeds it, and the order to fix it in if it hangs around.

Short answer: what to do first

Don't panic and don't start dosing anything. Brown diatom film is one of the most common things to show up in a new tank. Aquasabi and Buce Plant both say it usually appears about one to three weeks after setup. Aquasabi calls it a normal phase. Green algae often crowd it out a few weeks later, and after that it usually doesn't come back. Aquarium Co-Op calls it one of the simplest algae to get rid of, and Buce Plant says it "should go away in time as your tank balances out."

So your first job is simple:

• Wipe it off the glass.
• Brush it off the stones and wood.
• Siphon the loose film out during your normal water changes.
• Wait.

Hobbyists often say a bloom took two or three months to clear completely. Those are anecdotes, not a rule, but they give you a sense of how long "wait" can mean. If you set your tank up this fall, there's a good chance the brown is gone by Thanksgiving.

Two things the film does not mean: it doesn't mean the tank has failed, and it doesn't mean the tank is cycled. Keep testing for ammonia and nitrite either way. The fixes further down only matter if the brown won't leave.

What diatoms are and why new tanks get them

Diatoms are tiny algae that build their cell walls out of silicate, so silicate is what they need to grow. A new tank often has plenty of it. Fresh sand, gravel and stone can release silicate, and Buce Plant notes that tap and well water often carry it. If your tap water is high in silicate, every water change tops it up.

A new tank also has very little competition. There are few plants, or the plants are small and still rooting in, and almost no other algae has settled in yet. Aquarium Co-Op says diatoms are most common in newly planted tanks and links them to high phosphate and silicate. Buce Plant adds that with too few plants and high nitrogen, diatoms thrive, and overfeeding only adds more waste.

What diatoms don't tell you is where your cycle stands. A tank usually takes around three to six weeks to cycle, depending on its size and how many animals are in it. Diatoms can show up before that finishes, at the same time, or after. Your test kit is the only thing that tells you the cycle is done.

In a river tank, this is the film you'll notice first. When I ran the Finn Strong River Tank kit in my 55-gallon long, the stones in those three pools and the driftwood were the whole point of the tank. A brown dust on them stands out more than it would on a plain back wall.

Clean it off by hand

Diatoms are soft and come off easily, which is the good news. Buce Plant's routine works well, top to bottom:

1. Glass: use a scraper with a long handle, an old gift card, or a magnetic cleaner.
2. Plant leaves: rub them gently between your fingers or with a soft toothbrush. Go easy on delicate leaves.
3. Stones and wood: scrub with a toothbrush. A wire brush works on hard rock, but keep it off driftwood you want to stay smooth.
4. Cleanup: the film you knock loose drifts down and settles. Siphon it out with a gravel vacuum during your water change, as Aquasabi suggests, so it doesn't just land somewhere else.

In a flowing river tank, scrub with the pump off for a few minutes. Otherwise the current spreads the loose film through every pool before you can siphon it.

Expect it to come back. Wiping only clears what you can see. If the silicate source is still in the tank or your tap water, the film can return within days. That's not a sign you're doing it wrong. Cleaning keeps the tank looking good while the source runs out, and the next section deals with the source.

Cut the silicate and balance the tank

If the brown keeps coming back for weeks, go after what's feeding it.

Test your tap water for silicate. If the brown keeps coming back, Aquasabi's first step is a silicate test. There's no single number that counts as too high for every freshwater tank, so compare your tap water with your tank water and look for a steady source. If your tap water is the source, Aquasabi gives two fixes:

• Mix in RO or DI water for part of each water change.
• Use silicate-removing filter media.

Both also lower phosphate. That's fine in a fish-only tank, but in a planted tank or riparium it can leave plants short of a nutrient they need, so watch them.

Add plants. This is Aquarium Co-Op's main fix: plants take up the extra phosphate and silicate, and the diatoms lose their food. Fast growers do this best. In a riparium, plants with their roots trailing in the water do the same job.

Don't overfeed. Leftover food breaks down into the nutrients diatoms use. Feed what your animals finish quickly.

Keep the light steady. Buce Plant recommends 6–8 hours a day, "no more than 8 hours and no less than 6." That's one retailer's guideline, and keepers disagree about whether more or less light helps with diatoms. A timer set to a steady schedule is still a sensible place to start.

Keep water changes moderate. Buce Plant recommends weekly water changes. Avoid huge ones while the tank is cycling. A saltwater-focused article from That Pet Place warns that changing more than about half the water at once can set off a diatom bloom even in an established tank. Its other advice applies to freshwater too: be patient and skip unnecessary chemicals.

Cleanup crew: who helps and when

Algae eaters can speed things up, but they can't fix the cause, and the wrong animal at the wrong time can get hurt.

Nerite snails have the best support. Aquasabi says nerites (Neritina, Vittina and Clithon species) "have proven to be very efficient" against diatom films. It suggests about one snail per 10 liters, which is roughly one per 2.5 gallons. It also warns that just a few snails won't make a noticeable difference.

Otocinclus, Amano shrimp and bristlenose plecos often appear on algae-eater lists. Aquarium Co-Op says otocinclus, snails and shrimp eat diatoms. Buce Plant lists bristlenose plecos, otocinclus, nerites, Amano shrimp and hillstream loaches. Aquasabi's algae-eater page names snails for diatoms, not shrimp, so treat the others as possible helpers rather than a sure fix.

Timing matters more than species. Don't add anything until ammonia and nitrite both read zero. Aquasabi says invertebrates can usually go in after about two weeks of cycling, once those readings are gone, and algae-eating fish after about a month. If one of these grazers is a schooling fish, like otocinclus, keep a proper group rather than one or two.

River-tank check: before you buy, make sure each animal fits your tankmates, your water temperature and your current. Buce Plant gives the same advice: research each animal's fit first. A crab, frog or turtle sharing the tank can see a small snail or shrimp as food. And if you keep crabs, use a full screen lid. That's the first lesson my river tank taught me.

When to worry

Most diatom blooms fade without help. Look closer if:

• the brown is still going strong well past two to three months, or
• it turns thick, slimy or stringy instead of dusty.

If it lasts that long, test silicate, phosphate and nitrate in both the tank and your tap water. Then think about what has changed: a new water source, a well, a new bag of substrate or rock, or extra feeding. A source you didn't know about usually explains a bloom that won't quit.

If the growth looks slimy or stringy, or peels off in sheets, it may not be diatoms. Cyanobacteria and other growths can look like it at first glance. Identify it before you treat it, because the fix for diatoms (patience, plants and wiping) may not be the fix for something else.

Sources (6)

• Aquasabi: Brown algae (diatoms)
• Aquasabi: Algivores (Oliver Knott)
• Aquarium Co-Op: Brown diatom algae FAQ
• Aquarium Co-Op: Diatoms
• Buce Plant: How to get rid of brown algae (diatoms)
• That Pet Place: Diatom algae in new tanks

Photo by PatrickChartol on Pixabay.