Purple Algae in Fish Tanks – What Can You Do About It?

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Algae are the curse of the aquarium keeper, often covering the viewing panes, decorations, and plants in the tank with a slimy coating that ruins the aesthetic of your aquascape.

In this article, we focus on two types of purple algae, one of which affects saltwater tanks and one afflicting freshwater environments.

One of these types of algae you want in your tank, whereas the other form of purple algae you definitely don’t need!

Read on to learn about these two forms of purple algae.

Coralline Algae

If you have a reef tank, Coralline algae are a very welcome resident!

How so?

Well, the presence of Coralline algae is a good indicator that your reef aquarium is healthy and stable. These algae add a beautiful natural feel to the ecosystem, as well as provide valuable structural support.

What Are Coralline Algae?

Coralline algae are a form of red algae belonging to the order Corallinales.

The red algae group is extremely diverse, with most varieties comprising multicellular marine organisms capable of sexual reproduction.

Some of these types of algae, including Laver and Dulse, are cultivated and harvested as a food source in some parts of the world.

The algae’s coloration comes from pigment accessory to the chlorophyll that Coralline algae use to photosynthesize.


Coralline algae are primarily marine forms of algae that have been around since the Cretaceous period.

All species of this form of algae, except one variety from Croatia, are marine, and a few can survive in brackish conditions.

The algae grow along intertidal shoreline zones, receiving alternate immersion and exposure to the air. You’ll find this species of purple algae growing in waters to a depth of around 270 meters.

Ocean water is essential for Coralline algae since the species needs the minerals in the water for the formation of the algae’s aragonite skeletons.


Although the algae need light, the amount of light required depends on the species of purple algae concerned.

Keeping these algae can be challenging since you need to provide light levels that suit the particular strain concerned.

For example, some varieties of Coralline algae can tolerate low-light conditions, whereas others need bright lighting.

Purple algae aren’t fussy about what substrate they have to bond with. The organisms will grow on stony corals, rocks, aquarium glass, the shells of snails, filters, and powerhead outflows.

That can be a pain if you don’t want your tank overrun by Coralline algae.

What Are the Benefits of Coralline Algae in a Reef Aquarium?

Unlike other species of nuisance algae, including hair algae, green algae, and brown algae, purple algae offer some benefits to aquarium owners.

Healthy Environment Indicators

The growth of Coralline algae is a strong indicator of a healthy environment in a reef tank.

That’s because this form of purple algae needs minerals, including calcium, carbonate, magnesium, and trace elements, to grow and remain healthy.

Stony corals need the same minerals and trace elements. So, if the algae are growing well, you know that your corals have everything they need, too.

Binding Agents

Coralline algae help to stabilize natural and manmade reefs, sealing cracks in the substrate and enhancing the overall stability of your hardscape like a form of natural mortar.

Controls Nuisance Algae

Coralline algae colonies remove free-floating nutrients from the water that would otherwise be used to fuel the growth of less desirable algae.

Most species of Coralline algae can shed their outer shell periodically to get rid of nuisance algae bloom that might block out the light if its growth is uncontrolled.

Provides a Healthy Attachment Site

Corals and some other sessile organisms often choose to attach themselves to Coralline algae-encrusted rocks.

It’s assumed the long-lived algae indicates a healthy site that the coral can benefit from sharing.

Beautiful in Their Own Right

There are two main forms of Coralline algae.

Geniculate Algae

Geniculate algae form tree-like colonies that can make an attractive architectural feature in your reef tank.

Non-geniculate Algae

Non-geniculate algae grow in a crust-like form, creating patches of color or structures that resemble coral growth.

Coralline algae come in many color variations, including:

  • purple
  • orange
  • red
  • pink
  • green

Many times, the pop of color provided by this beautiful algae outshines the hues of your corals!

Food Source for Invertebrates

Coralline algae can provide a valuable food source for many invertebrates, including hermit crabs and sea urchins.

Fish, such as tangs, chitons, parrotfish, and limpets, also eat purple algae.

What Are the Downsides of Purple Algae in Your Marine Tank?

Unfortunately, despite its benefits, purple Coralline algae can be a nuisance, too.

The main problem with this species of algae is its habit of growing on any hard surface, even your aquarium viewing panes.

Although a coating of purple algae across the rear of your tank can look stunning, if the algae spread to the front of your tank, you’ll lose your view!

Although Coralline algae can be useful in binding together live rock and decorations, it can be a pain if your decor finishes up stuck to your filter and your viewing panes.

How To Get Rid of Coralline Algae

Like corals, Coralline algae grow an aragonite skeleton, so you can’t simply brush the colonies away. So, you need to prise the algae off its chosen attachment point with a flat blade or a plastic credit card.

Start by soaking encrusted equipment and decorations in a 1:20 vinegar-water bath. That will dissolve the algae’s carbonate skeleton, making removal easier.

You can use the same strategy on encrusted viewing panes.

As previously mentioned, Coralline algae are excellent barometers of a healthy environment in your reef tank. However, the organisms can also become competitors for nutrients required by your corals.

So, you must closely monitor the water chemistry to ensure that both organisms receive what they need.

Can I Buy Coralline Algae?

Coralline algae often seem to mysteriously arrive in an aquarium, even though the spores don’t float freely. So, this algae comes from an external source.

Live Rock

Coralline algae are often imported into a new reef tank on live rock. So, if you want to establish colonies of purple algae in your aquarium, look for a piece of rock that already contains visible algae.

You often see hermit crabs carrying patches of Coralline algae on their shells, and that can be another good way of introducing the algae into your tank.

Donation From Another Reef Aquarium

If you can find another reef tank enthusiast or a pet store with Coralline algae colonies in one of their tanks, you could ask for scrapings of algae from a piece of equipment or from the glass.

All you have to do is put algae scrapings into your aquarium, ideally in a spot that receives moderate light.

Liquid Infusion

You can buy liquid infusions of Coralline algae from some specialist suppliers.

Kent Marine Tech Bottle Purple

  • Contains a natural plant growth regulator for purple coralline and green calcareous macro-algae
  • Provides specific concentrations of calcium, magnesium, strontium, trace minerals and carbonates
  • Formulated with precise ratios of natural seawater components
  • Not for human consumption

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Alternatively, if you can get hold of a donation from another hobbyist, you can create your own liquid infusion.

Mince the algae scrapings in water to create a smoothie, and add the mixture to the water column close to your powerhead or filter outflow.

Coralline Algae Care Guide

Coralline algae are pretty straightforward to care for. The organisms prefer clean water with plenty of calcium, carbonates, and trace minerals to fuel growth.

Phosphate levels should be kept low since an excess can retard Coralline algae growth while encouraging the growth of nuisance algae types.

Water Parameters

Coralline algae need a water temperature of between 78 to 82°F. The pH level should be between 8.1 and 8.3, with a calcium level of 400+ mg/l and carbonate hardness of 7-10dkh.

Ideally, phosphates should be at 0 ppm with nitrate levels below 5 ppm.

Those conditions are pretty standard for most reef tanks, but Coralline algae need stability over everything else.

Any fluctuations in water conditions and chemistry will cause the algal spores to remain free-floating, and any existing colonies won’t spread.

Coralline algae need iodine and strontium for good growth, so we recommend periodically adding an accelerator product to your tank water, such as CaribSea Purple Up Algae Accelerator.

Lighting Requirements

There are hundreds of different species of Coralline algae, which makes it difficult to recommend precise lighting requirements.

For that reason, we recommend seeding your aquarium with several types of Coralline algae. That enables the species best suited by your light levels to flourish naturally.

However, your light levels should be no more than 5 watts per gallon, ideally with blue actinic light, which encourages growth in most types of Coralline algae.

Growing Coralline Algae

After you’ve introduced Coralline algae to your tank and established the correct conditions, it’s really a matter of being patient and keeping the environment stable.

Most aquarists using CaribSea Algae Accelerator report rapid growth within a week or so of introducing the algae. However, if you don’t use an accelerator, you might have to wait 5 to 8 weeks or even longer for a crop of Coralline algae to grow.

Initially, you’ll notice small green or white patches on your tank glass or live rock. Those cells of Coralline algae eventually solidify into calcified algae of a purple or pink hue.

Depending on the species, you might need to scrape your viewing panes every week to keep the algae under control. However, other species might only produce a few centimeters of growth every year.

Black Beard Algae

Black Beard algae or Audouinella is a freshwater alga that can appear as a black, dark-purple color and is sometimes referred to as purple algae or Brush algae.

This common type of algae is a particular nuisance once it gets into your aquarium, growing on the tank walls, decorations, the substrate, and along the leaf edges of plants such as Anubias and Java fern.

Not only does Black Beard algae look unsightly, but it will eventually deprive your plants of light and nutrition, ultimately killing them.

Unlike Coralline algae, which is an indicator of a healthy reef tank, Black Beard purple algae is a nuisance algae species that you definitely don’t want in your aquarium.

How Do Black Beard Algae Get Into Your Tank?

Black Beard algae can find its way into your fish tank in two primary ways.

With New Plants

If you buy new aquarium plants, it’s essential that you disinfect the plants by rinsing them in a mild solution of bleach and water before adding them to your main display tank.

Black Beard algae spores are often attached to plants and are so tiny that they’re invisible to the naked eye.

Those spores rapidly proliferate, quickly spreading throughout your aquarium.

With New Fish

Although algae won’t grow on fish scales, Black Beard algae can get into your tank with new fish.

How so?

Well, the algae can hitchhike into your tank as free-floating strands in the bag used to transport the fish.

Once the algae are in your aquarium, it will quickly establish colonies and spread throughout the environment.

What Causes Purple Algae?

Purple Algae in Fish Tanks

There are a few factors that encourage the growth of purple or Black Beard algae.

Excessive Light

Algae is basically an aquatic plant that needs light to thrive and spread. So, provided the algae has plenty of light, it will spread extremely rapidly.

Algae especially love direct sunlight, so be sure to put your fish tank in a spot that receives no or very minimal sunlight.

If you have live plants in your tank, they will need light to grow. However, to keep purple algae at bay, you’ll need to restrict the amount of time that you have your tank lights on.

Ideally, you should limit the light in your tank to a maximum of 8 hours per day. Although that won’t eliminate the algae altogether, it will certainly help to retard its growth rate to controllable levels.

If you’re not around to switch the lights on and off, we recommend using an automatic light timer that you can preset.

Low CO2 Levels

If CO2 levels in the tank are low or unstable or your water circulation is inadequate, Black Beard algae will thrive.

Whereas regular aquatic plants need plenty of CO2 to thrive, algae don’t, so when levels are low, a purple algae explosion can be triggered.

Sometimes, carrying out a major water change can deplete CO2 in the water to such an extent that an outbreak of Black Beard algae happens. If you have a CO2 canister, double-check that it’s not running low and that the dose you’re using is correct.

Alternatively, we recommend that you consider adding a high-quality liquid carbon product to the water. That will elevate the carbon levels slightly, and it’s also lethal to purple algae.

Seachem Flourish Excel liquid carbon is an excellent product that’s used by many aquascapers and experienced aquarists.

Seachem Flourish Excel Bioavailable Carbon

  • CARBON PRODUCTION: All plants require a source of carbon, and Flourish Excel is a great and convenient source of bioavailable organic carbon.
  • IRON PRODUCTION: Flourish Excel also possesses iron reducing properties, thus promoting the ferrous state of iron (Fe+2), which is more easily utilized by plants than ferric iron (Fe+3).

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We earn a commission if you make a purchase, at no additional cost to you.

Can Purple Algae Harm Your Fish?

Fortunately, purple algae are not dangerous or harmful to your fish. In fact, the algae tend to grow in flowing masses, where fish and fry can take shelter.

Some fish species graze on the algae as a valuable part of their diet.

Many hobbyists like to keep a small number of purple algae, as it can look quite attractive when growing on driftwood and rocks.

However, if you decide to do that, you must be prepared to carry out regular maintenance to prevent the algae from taking over your whole aquarium!

How To Get Rid of Purple Black Beard Algae

There are a few effective methods of getting rid of Black Beard algae.

Trim Plant Leaves

Since Black Beard algae love to grow on plant leaves, the best way to get rid of it is to trim off any leaves where the algae have started growing.

Use aquascaping scissors to nip off the leaves cleanly and without tearing them.

Clean Viewing Panes and Decorations

Conventional algae magnets are pretty effective at removing purple algae from your tank glass. I use an algae magnet to clean the tank glass each week as part of my regular aquarium maintenance routine, which works pretty well.

If you have Brush algae growing on decorations, rocks, and driftwood, you can remove the item from the tank and scrub the algae away using an old toothbrush or wire brush.

If possible, it’s better to clean the decorations outside of the tank; otherwise, loose strands of algae tend to float away and set up new colonies elsewhere.

Once you’ve removed the algae, it’s a good idea to soak your decorations briefly in a weak bleach solution for a few minutes to kill off any remaining organisms.

Be sure to rinse all the items thoroughly before returning them to the aquarium.

CO2 Painting

If you have purple algae growing on your viewing panes or rocks, you can lower the water level to expose the algae to the air.

Now, use a paintbrush to apply Seachem Flourish Excel directly to the algae at the manufacturer’s recommended rate. After a minute or so, refill the tank to the usual level.

Within a couple of days, the algae should turn pink and begin to die off. As soon as that happens, you can use a scrubbing brush or old toothbrush to get rid of the dead organisms.

Use Algae Eaters

A few fish species eat purple Black Beard algae, including the Florida Flag fish and Siamese Algae Eater (Crossocheilus siamensis).

However, you’ll need to check that you have the correct species of algae eater since some fish stores sell a different species under the name of Siamese Algae Eater.

Unfortunately, there’s no guarantee the algae eaters will eat the algae! That’s often the case if you feed commercially prepared fish food to other species in a community tank.

A small group of shrimp will graze on the algae, although the shrimp won’t make much of an impact on a severe infestation.

Copper Treatment

If the purple algae infestation is severe, you could try treating your tank with copper. However, you must follow the manufacturer’s instructions in the letter and be aware that copper can adversely affect some fish, plants, and invertebrates.

Hydrogen Peroxide

You can use hydrogen peroxide to remove purple algae from your tank decorations and plants.

Take all your algae-infested decorations and plants out of the tank. Make up a solution of three percent hydrogen peroxide and water and soak the infested items in that for around three minutes.

Rinse everything thoroughly in clean water to get rid of the remaining hydrogen peroxide, and then replace the plants and decorations in your aquarium.


Vinegar can also be effective at killing Black Beard algae.

Start by removing all infected decorations and plants from the aquarium. Soak the items in a solution of one-quarter vinegar to three-quarters water for around three minutes.

Rinse the items thoroughly in clean water, and return them to your fish tank.

When treating plants, don’t immerse the roots! All you need to do is dip the leaves in the water and vinegar solution. Again, be sure to rinse the plants thoroughly before returning them to your tank.

Final Thoughts

Did you enjoy our guide to freshwater and saltwater purple algae? If you found the information helpful, please take a moment to share it!

If you have a marine or reef fish tank, Coralline algae are one of the most desirable algae strains you can have growing in your aquarium. The presence of these organisms shows that the environment in your fish tank is healthy and correct for your corals.

In addition, Coralline algae are very attractive and can help to provide a stable base for corals and live rock hardscapes.

However, purple algae in a freshwater fish tank are usually down to a Black Beard algae infestation. Black Beard algae can quickly overcome your tank, coating any solid surface with thick masses of dark purple slimy growth.

Fortunately, you can remove purple algae pretty easily by following one of the methods outlined above or using a combination of the most suitable for your setup.

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