Controlling Diatoms | Aquarium | Causes & Solutions | Tips from the Pros
How to Control and Remove Diatoms in Your Aquarium—Tips from an Expert
It’s a sight you never want to see in your aquarium: within a short time, diatoms cover aquatic plants, decorations, glass, and the substrate with a slimy, brownish film. In this guide, you’ll learn what diatoms are, how to prevent them, and what to do if diatoms have infested your aquarium.
Identifying Diatoms
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Diatoms are tiny single-celled aquatic plants that can appear in both freshwater and saltwater aquariums. There are thousands of species, and as protoplankton, they form the foundation of every food chain on Earth. Without diatoms, there would be no higher life—and they’re found almost everywhere. Individual diatoms are so small that you wouldn’t even be able to see them with the naked eye. It’s only when these “protozoa” clump together to form a coating and cover the bottom or the glass of the aquarium like a slimy mat that you notice them. Although they are distinctly brown in color, the term “brown algae” is incorrect—this type of algae lives only in seawater, whereas diatoms thrive anywhere the conditions are right. This can be freshwater, seawater, or even moist forest soil. The term “slime algae” is also incorrect, even though it fits very well: this term refers to blue-green algae, which, in turn, aren’t actually algae at all, but a type of bacteria. Very confusing, isn’t it?
| Diatom | Brown algae | Blue-green algae/slime algae |
| Single-celled organisms, protoplankton | Multicellular organisms, often large seaweed plants | Cyanobacteria |
| live in fresh and saltwater | live almost exclusively in saltwater | live in freshwater and seawater |
| slimy, slippery coating | Formation of leaves and shoots | slimy, slippery coating |
| brown | brown | greenish-blue, unpleasant fishy odor |
| harmless | non-toxic | Toxic in large quantities |
| Common during the initial phase | rather rare |
Causes of diatoms in the aquarium: How to prevent them
The name “diatom” is significant: diatoms incorporate silicic acid (a compound of silicon and oxygen) into their cell walls, which gives them stability. The name also hints at what these algae need most: silicic acid. This brings us straight to the conditions you need to prevent diatoms in your aquarium.
Unlike red algae or blue-green algae, diatoms appear at a specific time in almost every aquarium: namely, during the cycling phase. In a tank freshly filled with tap water, if nutrient levels haven’t yet stabilized—because the plants haven’t taken root yet and the filter bacteria haven’t started working—there’s often an excess of silicic acid. Silicic acid is present in tap water and, over time, converts to silicon dioxide in the aquarium. This form can no longer be utilized by diatoms—they simply disappear.
The first and simplest cause of diatoms in the aquarium is therefore simply an excess of silicic acid in the fresh water, which resolves itself over time. Additional preventive measures you can take against diatoms in the aquarium are actually a matter of course for any good aquarist:
Preventing diatoms
- Use plenty of fast-growing plants to compete for nutrients
- Check water parameters regularly, especially for phosphate
- Perform regular partial water changes
- Don’t set the lighting duration too short
- Consider using a reverse osmosis system (filters out silicic acid) or
- Use a silicate absorber in the filter
Unlike many other algae, diatoms do not particularly like light—they prefer to settle on darker substrates or in the shade of aquatic plants. Therefore, ensuring sufficient light with a new fluorescent tube is also a preventive measure against diatoms.
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Measures Against Diatoms in the Aquarium
Reducing Silicic Acid
While tap water is the main source of excess silicic acid in uncycled tanks, this is not the case later on when performing regular partial water changes. In these cases, you can usually use tap water without concern, as the filter bacteria quickly break down the silica. If the silica content is still too high and leads to a diatom infestation, you can add reverse osmosis water from a reverse osmosis system (never fill the entire aquarium with it!) or simply use water from another aquarium that is free of diatom infestations.
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You can also carefully lower the pH level to combat diatoms. However, only do this if your fish can tolerate it.
A functioning filter is crucial for the rapid removal of silicic acid from the water. Once your filter has developed a healthy population of filter bacteria, you should never clean it so thoroughly that this bacterial layer disappears. In the initial phase, you can boost the growth of filter bacteria by “inoculating” the filter with an initial batch of bacteria. A silicate absorber (also known as a silicate adsorbent), which removes silicic acid from both freshwater and saltwater, is effective both as a preventative measure during the start-up phase and for the acute treatment of a later diatom infestation.
Reducing Phosphate Levels
Diatoms thrive in high phosphate levels. In aquariums, this problem usually occurs when fish are overfed and do not eat all their food, causing it to sink to the bottom. Dead fish or snails that you don’t discover quickly enough, or frozen food that hasn’t been rinsed thoroughly, also introduce large amounts of phosphate into the tank. Therefore, it’s important to regularly measure the key water parameters and monitor their trends. If the phosphate level spikes (more than 0.25 mg/l is too high), you can take immediate action—for example, by adding a phosphate absorber. If your tap water already contains too much phosphate, installing a reverse osmosis system can help.
An elegant biological solution is the ivy plant: This houseplant, well-known as an office plant, consumes a great deal of phosphate. Let one of its roots dangle into the aquarium, and your problem will solve itself.
Controlling Diatoms with Light
Since diatoms tend to grow at the bottom of the aquarium and in shaded areas, people often assume that they prefer darkness. But that’s not actually true. Rather, it’s more accurate to say that diatoms can thrive even with less light—unlike other types of algae. That’s why they primarily settle in places where other algae don’t grow. You should be very careful about extending the lighting duration, as this can in turn encourage the growth of other algae. It might be helpful to let more light into the shaded areas of your aquarium—the very places where the diatoms grow.
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Removing diatoms by hand
Fortunately, the unsightly, slimy diatom deposits can be removed quite easily by hand, since—unlike red algae—they do not adhere very firmly to the substrate. You can simply wipe diatom deposits off rocks or the leaves of aquatic plants by hand. An algae scraper is very handy for the aquarium glass, since you don’t have to reach into the water. You can vacuum diatoms off the substrate with a hose. After you’ve removed the diatoms, a partial water change is recommended; otherwise, they’ll quickly spread again. And, of course, removing diatoms by hand will only be effective in the long term if you also adjust the conditions in the aquarium.
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Summary
If you notice a slimy brown film in your aquarium, it’s likely diatoms. Once you’ve confirmed beyond a doubt that it’s not blue-green algae or brown algae, you can take targeted action against the diatoms. Unless, of course, your aquarium is currently in the cycling phase; in that case, the diatoms will usually disappear on their own after a few days once they’ve absorbed the excess silicic acid in the water. Even if an infestation occurs later on, you can get diatoms under control quite easily—reduce the levels of silicic acid and/or phosphate in the water, and the slimy deposits will soon disappear again.
