Brown diatom algae is the algae that panics the most people and needs the least intervention. It is the clearest sign that an aquarium is still finding its feet, and in almost every case it resolves on its own. Understanding why tells you both how to speed it up and how to avoid it altogether next time.
What are diatoms?
Diatoms are single celled organisms that build themselves a rigid shell, called a frustule, out of silica. They coat glass, substrate, hardscape and plant leaves in a soft brown or tan dust that can appear almost overnight and spread across an entire aquarium within days.
Diatoms in a newly set up aquarium. The brown coating spreads across every surface but wipes away with almost no resistance.
They are easy to identify by feel. Diatoms come away with almost no effort, and if you rub the residue between your fingers it feels faintly gritty. That grit is the silica shell, and nothing else in a freshwater aquarium behaves that way.
They are harmless to fish, shrimp, snails and plants. Heavy coverage on a leaf can block enough light to weaken it, but the diatoms themselves do no direct damage.
Diatoms, green dust or cyanobacteria?
What causes diatoms?
A biologically immature aquarium
This is the overwhelming cause. Diatoms thrive in the unstable conditions of an aquarium whose biofilter has not yet established, where ammonia and organics fluctuate and no mature biofilm is competing for surfaces.
Here is the distinction that trips most people up: cycled and biologically mature are not the same thing. A cycled aquarium can convert the ammonia it is currently producing. A mature one carries a dense, diverse microbial population across every surface in the tank, and that takes months rather than weeks. Your ammonia and nitrite can read zero, your fish can be perfectly healthy, and the aquarium can still be nowhere near mature. Diatoms are frequently the visible sign of exactly that gap, which is why they so often appear in a tank the owner is certain has finished cycling.
A damaged or crashed cycle
Established aquariums get diatoms too, and it almost always follows something that knocked the bacterial colony back:
- Rinsing filter media under chlorinated tap water, which kills beneficial bacteria on contact
- Replacing too much filter media at once
- Over cleaning, particularly a deep substrate clean and a filter service in the same session
- Medicating the aquarium, especially with antibiotics
- An extended power outage leaving the filter without oxygen
If diatoms appear in an aquarium that has run cleanly for a year, look at what changed in the fortnight before they arrived.
What about silicates?
A common claim is that diatoms are caused by high silicate levels, usually blamed on a sand substrate. This is worth handling properly, because the popular answer is not quite right in either direction.
Diatoms genuinely do need silica. They build their frustules from it and cannot grow without it. That part of the claim is entirely true.
The problem is that silica is rarely the limiting factor. Mains water across Australia carries dissolved silica well above what diatoms require, and it arrives with every water change regardless of what your substrate is made of. Aquarium sand, meanwhile, is generally quartz, which is extremely insoluble and releases very little into the water column.
So in most aquariums silica is a requirement for diatoms rather than the trigger. A requirement that is already abundantly met cannot explain why one aquarium gets diatoms and the one beside it does not. The variable that actually differs between those two tanks is usually biological maturity, which is why diatoms track tank age far more reliably than they track substrate type.
Do silicate removers work?
They can. Silica is genuinely required, so stripping it out of the water column does limit diatom growth, and in a tank where you need visible improvement quickly a silicate resin will often deliver it.
Treat it as a bandaid rather than a cure, though. The resin is managing the symptom while the underlying cause, an aquarium that has not finished establishing, carries on unchanged. Once the media exhausts or you take it out, diatoms will return unless the tank has matured in the meantime. If you have a genuinely high silicate source, or an aquarium you need presentable for a specific reason, a remover is a reasonable tool. For a new tank that simply needs a few more weeks, your money is better spent elsewhere.
How long do diatoms last?
Most outbreaks run two to six weeks and then stop, often quite abruptly. As the biofilter matures and biofilm establishes across the aquarium's surfaces, diatoms lose their advantage and disappear.
The two lines are the same story told twice. Diatoms fade as the bacterial colony fills out, which is why treating them chemically achieves so little.
If you have had diatoms for more than two months, something is holding the aquarium back. Check whether you are over cleaning the filter, whether the tank is genuinely cycled, and whether an ongoing ammonia source is present.
How to get rid of diatoms
The goal is to physically export diatoms while you wait for the aquarium to mature. You are managing the symptom rather than curing anything, and that is the correct approach here.
The best method by a wide margin. Diatoms lift straight off substrate and hardscape with a siphon, and this genuinely removes them from the system rather than redistributing them through the water.
50% or more during the diatom phase. Each change gives you another opportunity for manual removal and dilutes the organics feeding them. Always use dechlorinated, temperature matched water.
Work gently and hold the siphon right beside the brush, so what you dislodge leaves the aquarium rather than settling somewhere else. On glass, any sponge or magnet will do, since diatoms come off with no effort at all.
This is genuinely part of the treatment rather than a consolation. Keep up the manual removal, look after your plants, and the tank will grow past it.
What not to do
Two ways to turn three weeks into three months
- Do not treat them chemically. Diatoms are a symptom of an aquarium that has not finished establishing, and knocking them down does nothing about that. Worse, it risks disturbing the bacterial colonisation you are waiting on, which prolongs the exact condition causing the problem.
- Do not over clean the filter chasing them. This is the most common way people extend a diatom phase. Stripping and scrubbing the filter every week because the tank looks dirty sets the bacterial population back each time, which is precisely what keeps diatoms in business.
- Do not stock heavily to fix it. Adding a few grazers to a cycled aquarium is fine and genuinely helps. Loading a full stock into a tank that has not finished cycling puts the livestock at risk and raises the ammonia and organic load, which makes the diatoms worse rather than better.
How to prevent diatoms entirely
This is where the real answer lies, and it is a setup decision rather than a treatment.
Dark start your aquarium
A dark start makes diatoms close to a non issue in a new aquarium, and it is the single most effective thing you can do about them.
The reason it works is worth understanding rather than just following. A new aquasoil releases a substantial pulse of ammonia across its first weeks, and a new filter takes weeks to build a working bacterial population. Both of those are exactly what diatoms exploit. Running the aquarium in complete darkness lets both processes finish while there is no light available for anything photosynthetic to take advantage of. By the time you switch the lights on, the ammonia pulse is over and the biofilter is established, so the window diatoms need has already closed.
Your plants also go into an aquarium that is already cycled and stable, which means faster growth from the first day rather than a month of sulking.
The full method, including how long to run it and how to tell when it is finished, is covered in how to dark start and cycle an aquarium.
A dark start needs three things. AquaLabs Vitality to remove chlorine and chloramine from every drop going in, AquaLabs Nitro Cycle to seed the filter with heterotrophic and true nitrifying bacteria, and AquaLabs Fishless Start to supply a controlled ammonia dose for the colony to feed on.
If the aquarium is already planted and running
A strict water change schedule is the next best thing:
- Week 1: 50% daily
- Week 2: 50% every second day
- Week 3: 50% twice
- Week 4 onward: weekly
At this volume of water changing, untreated chlorine or chloramine will do real damage to the bacterial population you are trying to grow, so dechlorinate everything.
Never rinse filter media under the tap
Rinse mechanical media in a bucket of old tank water instead. If you must use tap water, treat it first and let it stand.
Discard old and decaying growth
Plants smothered by diatoms will struggle. Trim off the healthy new growth to replant, and discard old decaying stems rather than leaving them to break down and add to the organic load.
What eats diatoms?
Diatoms are soft and easily grazed, so almost every algae eater will take them. Effectiveness varies considerably.
- North Queensland and Darwin algae eating shrimp. The most effective shrimp available to Australian aquarists, and well worth seeking out. Amano shrimp are recommended everywhere online but cannot be legally imported into Australia, so these natives are the local equivalent and every bit as good at the job.
- Otocinclus. Outstanding on diatoms and the best fish for the job. One caution: otos need an established biofilm to feed on and they starve easily in a brand new aquarium, so do not add them in week one specifically to fix this.
- Nerite snails. Reliable diatom grazers, particularly on glass and hardscape.
- Bristlenose catfish and Siamese algae eaters. Both will clear diatoms readily.
- Cherry and crystal shrimp. They will graze diatoms, but far less effectively than the algae eating species above. Choose them because you want them, not as an algae solution.
If your aquarium is cycled, adding a few grazers while diatoms are present is perfectly reasonable and will help. Stock sparingly rather than all at once, since the tank is still building the biological capacity to handle a full load.
The short version
- Diatoms are a sign of a biologically immature aquarium, and cycled is not the same as mature
- Silica is required by diatoms but rarely the limiting factor, so it is seldom the real trigger
- Silicate removers work as a bandaid, not a cure. The tank still needs to mature
- Sand substrate does not cause them. Quartz sand releases almost no soluble silica
- Most outbreaks run two to six weeks and then stop on their own
- Siphon them out and do large water changes, always dechlorinated
- Do not treat them chemically and do not over clean the filter
- A dark start prevents them almost entirely, and gives your plants a better start too
- North Queensland algae eating shrimp and otocinclus are the most effective grazers
- If they persist past two months, the aquarium is not truly cycled or is being over cleaned
Diatoms look alarming and mean very little. They are the aquarium equivalent of growing pains, and the aquarium grows out of them.