How to Control and Remove Brush Algae | Aquarium | Causes & Solutions | Tips from the Expert
How to Control and Remove Brush Algae (Red Algae) in Your Aquarium—Tips from an Expert
- 1.Identifying Brush Algae
- 2.Causes of brush algae in the aquarium: How to prevent it
- 3.Measures against brush algae in the aquarium
- 3.1Controlling Brush Algae: Lighting and Water Flow
- 3.2Combating brush algae naturally: Predators
- 3.3Removing brush algae by hand
- 3.4Chemical treatments for brush algae
- 4.Summary
The aquarium has just been set up, the fish seem to be doing well—and then this happens: Ugly, hairy algae spread like wildfire over the aquatic plants and substrate, undoing all your hard work. Now you’re at a loss—because you need to act fast! In this post, we’ll show you how to identify the dreaded brush algae in your aquarium and how to successfully (and with a little luck) get rid of it without harming your fish or aquatic plants.
Identifying Brush Algae
Unlike many other algae, brush algae are relatively easy to identify. This is because they have a very distinctive growth pattern: as soon as the algae are a few days old, they form tassel-like, relatively short hairs or filaments that look exactly like little brushes. They also feel significantly stiffer than, for example, the slimy filaments of thread algae (which, incidentally, also grow much longer). Brush algae differ from their close relatives, beard algae, in the length of their filaments: They are significantly shorter than the filaments of beard algae and do not curl.
Along with beard algae, brush algae belong to the group of red algae. Interestingly, red algae do not look red at all, but rather dark green, gray, or almost black. The colloquial name for these pesky aquarium inhabitants is therefore “black algae.”
This type of algae is found primarily in the ocean, though some species also prefer freshwater—and they eventually colonize almost every aquarium because they can spread very quickly. All it takes is a single contaminated net from a pet store or an aquatic plant from a friend that’s still visibly uninfected to introduce brush algae into your aquarium.
You can determine whether you’re actually dealing with brush algae using a very simple test: Soak the algae in high-proof alcohol for a few hours. If they turn bright red, you’re dealing with red algae (hence the name). A red alga with shorter, tassel-like tufts is almost certainly brush algae. If the alga’s filaments are significantly longer and even curl, it is beard algae.
It becomes difficult to identify when brush algae are just beginning to form: their growth phase starts as small dots. If these are attached to the glass or on the leaves of aquatic plants, they are often mistaken for dot algae (from the green algae family). You can scrape off dots attached to the glass—if this is very difficult, they are likely brush algae.
TABLE: Identifying brush algae
| Bristle Algae | Beard algae | Thread algae |
| Group of red algae | Group of red algae | Group of green algae |
| Short, tassel-like filaments | Filaments up to 15 cm long and curled; the longer they are, | Filaments up to 50 cm long, smooth |
| Filaments are hard and non-detachable | Filaments are hard and non-detachable | Filaments soft, easily detachable |
| Color: dark green, gray to black | Color: dark green, gray to black | Color: grass green to deep green |
| Alcohol test: turns red | Alcohol test: turns red | Alcohol test: no color change |
Causes of brush algae in the aquarium: How to prevent it
Both beard algae and brush algae are extremely stubborn once they’ve taken hold in the aquarium. Unlike green algae, red algae cannot be easily removed because their basal cells attach extremely firmly to the substrate. You can tear off the tufts and filaments, but some basal cells will always remain, from which new algae will quickly grow. On top of that, brush algae (just like beard algae) have a clever trick up their sleeves: When there’s little carbon dioxide in the water and taller aquatic plants grow more slowly or die off, red algae can extract CO₂ from the carbonate hardness in the water—a process known as biogenic decalcification. They then store the byproduct, calcium carbonate, in their cells, which makes them hard—and inedible for most fish, shrimp, and snails. Even species known as algae eaters give brush algae a wide berth.
The best way to combat brush algae is therefore to prevent them from entering your aquarium in the first place. Strict cleanliness is the top priority:
- Buy fish and aquatic plants only from specialty retailers where you see absolutely no algae in the tanks.
- Carefully inspect new aquatic plants for algae and remove any infested leaves; if necessary, remove the entire plant.
- Substrate and hardscape should be thoroughly washed before being added to the aquarium.
- Never buy used decorative items or hardscape; if you do, boil and scrub them several times.
- Do not allow mulm to accumulate in the substrate; vacuum it out regularly.
- Frequent partial water changes reduce the number of bacteria, excess nutrients, and algae spores in the water.
As we’ve already mentioned, all these precautions often don’t help anyway, since algae always find a way to grow. But that’s not a problem as long as there’s a balance in your aquarium. If your aquatic plants receive enough nutrients to grow well, algae—as direct competitors for food—won’t have a chance to spread excessively. If you also keep fish or shrimp that enjoy eating algae as a snack, these lowly aquatic plants won’t be able to spread. If you see small tufts of brush algae on the filter outlet or right next to it in the current (that’s where they like to be), that’s still no reason to panic. As long as you keep an eye on your aquarium’s parameters, these little companions can stay there—they just need to be prevented from spreading to the rest of the aquarium.
Checklist: Preventing brush algae
| Check | Cause | Fix |
| Enough plants? | The more plants, the more competition for brush algae | Combine many fast-growing plants from all growth zones; boost growth with carbon fertilizer if necessary |
| Is the carbon dioxide level in the water high enough? | Plants grow more slowly, which makes it easier for brush algae to thrive; biogenic decalcification makes the water undigestible for algae-eaters | Use enough plants, perform regular partial water changes, and use a CO2 system |
| Is the current too strong? | Bristle algae thrive in current | Possibly reduce the filter pump’s flow or reposition the filter; adjust the filter to the tank size; place the filter outlet underwater |
| Are there too few nutrients in the tank? | Thread algae utilize nutrients much more efficiently than aquatic plants; if the plants stop growing, the algae will spread more quickly | Add carbon fertilizer or liquid fertilizer to stimulate plant growth |
| Too many nutrients in the tank? | As soon as nutrients remain unused, they are utilized by brush algae for reproduction | Add more fast-growing aquatic plants, feed the fish less, and perform regular water changes |
| Are nitrate and phosphate levels too high? | Nitrate and phosphate are the main food sources for brush algae; concentrations that are too high harm both aquatic plants and fish | Feed the fish less, rinse frozen food thoroughly, vacuum mulm from the substrate, do not use activated carbon in the filter |
| Is the iron level too high? | Is the iron level too high? | Reduce the use of iron-rich fertilizers |
| Is the lighting balanced? | Algae love light; too much of it promotes their growth | Never place the aquarium in direct daylight; limit lighting to a maximum of 11 hours per day; shorten the lighting time or take breaks as needed; replace fluorescent tubes regularly |
Measures against brush algae in the aquarium
Bristle algae were probably in your aquarium from the start; you just didn’t see them or their tiny spores. Even a brief fluctuation in your tank’s nutrient balance—perhaps due to a temporary rise in water temperature or a change in food—can open the door for this extremely fast-growing brush algae: It spreads in the blink of an eye and colonizes aquatic plants, decorations, or even the glass. Brush algae prefer to settle in areas directly exposed to the current. But they can also attach themselves to the aquarium glass, where they are often mistaken for spot algae.
The problem with brush algae is twofold: First, no one yet knows exactly what causes brush algae to spread; there are only speculations at best. Second, and this is much more important for you, it takes a very long time for brush algae to respond to any control method. Since they store nutrients as reserves, it can take weeks or months before you notice whether the algae control measures are having an effect. Even dead brush algae remain firmly attached to the substrate for a very long time before they fall off. Since you’ve often tried all sorts of different things by then, it’s never clear in the end what actually helped. This also makes it very difficult, even among aquarists, to separate useful advice from useless tips.
Controlling Brush Algae: Lighting and Water Flow
One way to make life difficult for brush algae is to target their favorite thing: water flow. In the early stages, brush algae will disappear simply by reducing the flow in the aquarium slightly or changing the direction of the filter outlet. Often, this outlet is above the water or just slightly below the water’s surface; in that case, try moving it deeper underwater.
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Lighting can also be a decisive factor in the growth of brush algae. It goes without saying that your aquarium should never be placed in direct daylight or, even worse, sunlight. It may help to simply cover a side of the tank that’s too bright with plastic wrap or to change its location. Your fluorescent tubes shouldn’t be more than six months old. After this time, the light spectrum of these lamps changes without you being able to see it: Instead of primarily short-wavelength red light, they increasingly produce blue, long-wavelength light, which algae prefer for growth. Reducing the daily lighting duration is a very simple first step in the fight against brush algae that you should definitely try: It won’t harm the fish or aquatic plants if they get one hour less light. A “midday break” can also be effective, since brush algae aren’t adapted to sudden changes in light.
While the “dark treatment” is quite effective against blue-green algae and green algae, it seems to have almost no effect on brush algae.
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Combating brush algae naturally: Predators
Thanks to biogenic decalcification, the shaggy tufts of brush algae are quite tough and, unfortunately, are shunned by almost all known algae-eaters. Only the Amano shrimp and the piano snail are said to eat brush algae in some cases; this works better when the algae are still young. To make the job easier for natural predators, you can install a CO₂ system to increase the carbon dioxide content in the water. Without the calcium carbonate from the carbonate hardness, brush algae become softer and taste better.
Removing brush algae by hand
Removing brush algae by hand is quite difficult. First, the brush tufts are so short that they’re hard to grasp; second, the basal cells are so firmly attached that they’re almost impossible to remove. You can thoroughly scrub hardscape and decorations that can be removed from the tank and, if possible, boil them several times. However, manual removal can only ever be a supplement to other measures!
Chemical treatments for brush algae
Not many algae treatments are effective against stubborn brush algae. We’ve had good results with Microbe Lift Alg-Away, which contains a combination of salicylic acid and copper sulfate. However, copper can also be harmful to fish if dosed too high; so be sure to follow the manufacturer’s dosage instructions exactly.
Note that when calculating the water volume of your aquarium, you must not use the tank’s external dimensions as a basis. Substrate and hardscape take up a significant amount of space, so there is often considerably less water in the aquarium than you might expect. This frequently leads to overdosing with algae removers and fertilizers.
Decorative roots and plastic objects covered in brush algae—which would be damaged by this treatment—as well as aquatic plants can be soaked in a bath of carbon fertilizer (Happy Carbo or EasyCarbo) to remove the brush algae. It’s also possible to “mist” the tank directly with carbon fertilizer. Each day, work step by step on just a small area of the aquarium, treating it with carbon.
You must not use the highly effective carbon bath against brush algae on mosses!
One method we recommend as a last resort is hydrogen peroxide. This corrosive substance should be used as a 3% or 6% solution, and again, only applied selectively to specific spots via misting. Make sure that neither fish nor other aquarium inhabitants come into contact with the hydrogen peroxide! It can severely damage their gills and scales and even lead to their death weeks later.
No matter what you’ve used, you’ll know it’s working after a few days when the brush algae lighten in color, turning pink or even white. They now seem to taste better again and are eagerly devoured by algae-eaters.
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Summary
If you spot small, dark green to black tufts in your aquarium that look like brush bristles and feel slightly bristly, you’re most likely dealing with a brush algae infestation. This subspecies of red algae tends to settle in areas exposed to water flow and is extremely difficult to get rid of. The best defense against brush algae is therefore prevention: Ensure that brush algae (and other algae) don’t have a chance to multiply excessively by maintaining meticulous cleanliness, a balanced nutrient level, and plenty of fast-growing aquatic plants in the aquarium. If it’s too late for that, you should take a two-pronged approach: On the one hand, you need to combat the existing brush algae; at the same time, you must ensure that the basic conditions in the aquarium are balanced so that the brush algae stay away permanently or no longer spread excessively.
