Red Slime Algae: How To Remove and Prevent It

Jennifer Doll

Jennifer Doll


Red Slime Algae

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Have you noticed that your new substrate turning red? You may be dealing with red slime algae, which isn’t actually algae.

Instead, red slime algae is caused by a type of bacteria, cyanobacteria. Cyanobacteria is very common in new aquariums and naturally occurs in both marine and freshwater systems.

Keep reading to find out what cyanobacteria is, what causes it, and how to get rid of it in your aquarium! 

What Is Red Slime Algae?

Red slime algae is a misnomer for cyanobacteria. Cyanobacteria is the closest relative to algae by also photosynthesizing to create energy.

In fact, cyanobacteria is believed to have been the first organisms to produce oxygen over 3 billion years ago. This caused a drastic shift in the composition of aerobic and anaerobic life, known as the Great Oxidation Event

Through time and space, cyanobacteria have managed to survive harsh and changing conditions only to cause a headache for aquarium hobbyists.

Don’t worry though, it’s a natural part of the ecosystem and usually goes away on its own with time.

Identifying Red Slime Algae

Red slime algae looks exactly how it sounds: smooth, red, and covering everything.

The red variation of cyanobacteria is most commonly found in marine ecosystems while green cyanobacteria is more present in freshwater. Green bacteria is commonly referred to as blue-green algae due to its dark green coloration.

Red slime algae has a matting behavior. It typically grows on the substrate and lower regions of the tank where it can be peeled off in large chunks.

Oftentimes, there are bubbles trapped within this mat from photosynthesis which can cause a stringy effect. 

The red coloration is very dark and can appear brown as well. Red slime algae can sometimes be confused with coralline algae in its beginning stages to beginner hobbyists.

What Causes Red Slime Algae?

Red slime algae usually populate in the following weeks after a nitrogen cycle. Almost all new tanks experience cyanobacteria so it’s not something to be too worried about.

Cyanobacteria is mainly the result of excess nutrients and overall poor water quality in your tank. Water flow and incorrect lighting can also influence red slime algae to grow in areas with low current and wrong or irregular photoperiods

There are many reasons why a cyanobacteria outbreak could be happening in your aquarium, though it’s especially likely to grow in tanks that are less than a year old.

During this time, your tank is balancing beneficial bacteria populations and biodiversity as well as waste import and export. Water parameters are constantly shifting during this time, leaving advantageous marine life, like algae and cyanobacteria, to thrive. 

Is Red Slime Algae Harmful?

Red Slime Algae

Red slime algae can look horrible. There’s no nice way of putting it, this stuff gets everywhere and makes any reef tank look ugly.

However, do you need to be concerned about the safety of your fish and corals?

In the marine aquarium setting, cyanobacteria is not likely to harm your fish or invertebrates. However, cyanobacteria is harmful in nature.

Cyanobacteria blooms are more common in nature than they should be. These events happen when there are excess phosphate and nitrate levels, slow currents, and warm temperatures.

Most cyanobacteria outbreaks happen during late summer or early fall in freshwater, brackish, and saltwater ecosystems. Usually, these systems are influenced by fertilizers or run-offs which create the perfect conditions for bacteria to grow.

Some of the most known cyanobacteria outbreaks happen in Florida, where these events are regularly monitored via satellite imaging. Local warnings and advisories are issued during these times.

But what makes cyanobacteria harmful?

Like other harmful algae blooms, cyanobacteria blocks out competing species by stealing oxygen and nutrients. This can quickly lead to a massive loss of life, which in turn, results in increased decomposition.

Cyanobacteria also produces one of the most poisonous compounds on earth, cyanotoxins. Contact with these toxins can cause skin irritation, fever, vomiting, diarrhea, pneumonia, and even death. 

Cyanobacteria In the Aquarium

We know how devastating cyanobacteria can be in nature, but cyanobacteria is thankfully less extreme in your aquarium. 

It is important to note that not all species of cyanobacteria produce cyanotoxins, but there is no sure way of knowing. It is best to assume that you’re working with a toxic variety and to work quickly and efficiently to treat the problem.

In the meantime, you may lose a few fish. Algae eaters are especially susceptible during this time, as they may ingest the cyanobacteria but most species know to avoid it. 

For the most part, red slime algae is completely manageable and won’t crash your system. As long as it’s maintained, the problem should only last a few weeks at most.

Because of its toxicity, always make sure to use aquarium gloves and wash your hands thoroughly after working in a tank affected by cyanobacteria.

How To Get Rid of Red Slime Algae

As mentioned before, red slime algae is caused by several problems in saltwater aquariums.

The answer to getting rid of red slime algae lies in knowing what problem is causing it in the first place. Problems could include: 

Excess Nutrients

Excess nutrients are a major cause of cyanobacteria but usually work together with either poor lighting or low water flow.

It can be difficult to diagnose excess nutrients because, by the time the red slime presents itself, those excess nutrients have already been used and lowered to more normal values.

If you catch this problem early enough, you may even be able to trace your water parameters as they decrease while the slime increases.

For comparison, most saltwater tanks have <5 ppm nitrate and close to 0 ppm phosphate. Make sure that your source water and salt mix aren’t adding any extra nutrients!

Low Water Flow

Working with excess nutrients, low water flow can easily cause cyanobacteria and other pest algae to grow.

To determine if there are dead zones in your tank, simply trace how leftover food and waste move throughout the system. If you find that a lot is getting caught in a certain area, those sections may be more prone to growing cyanobacteria.


Usually, cyanobacteria is the result of too much light entering the aquarium, either from extended photoperiods or high intensities.

However, it can also grow in low intensities where it can outcompete other more light-demanding species. Each aquarium is unique with different lighting spectrums, so it’s important to note that photoperiods will vary.


Substrate shouldn’t cause too much trouble, but a general lack of substrate care and maintenance can help cyanobacteria to grow. 

Will Anything Eat Red Slime Algae?

It is not recommended to rely on a cleanup crew or fish to solve cyanobacteria problems. Remember, this bacteria can be toxic to fish and invertebrates so adding them isn’t always the safest method.

Instead, use cleanup crews and algae eaters to help prevent the problem before it happens. 

Not only do crabs and snails eat algae, but they also eat the detritus that cause algae. They are great options to add to your aquarium if you want to prevent the problem altogether. 

If you don’t have a cleanup crew and red slime algae has already started to form, then it’s better to manually solve the problem. The cleanup crew can always be added later.

How To Prevent Red Slime Algae

Though cyanobacteria is incredibly common in new aquariums, there are a few ways to prevent it from showing up in your aquarium.

Remember, the main causes for red slime algae are excess nutrients, low water flow, lighting, and substrate.

Control Aquarium Water Nutrients

Understanding water parameters and knowing how to control them can be difficult.

They can easily change from one day to the next, which can gradually lead to problems over time. These small fluctuations can be especially difficult for beginner hobbyists that might be new to the saltwater side of the hobby.

In general, you always want to maintain these constant values:

  • Temperature: 72-82° F (22.2-27.8° C)
  • pH: 8.0-8.4
  • Ammonia: 0 ppm
  • Nitrite: 0 ppm
  • Nitrate: <20 ppm
  • Phosphate: <0.05 ppm
  • Alkalinity: 8.0-12.0 dKH
  • Salinity: 1.020-1.025

The easiest way to control water parameters is by doing regular partial water changes.

Most hobbyists perform 10-25% water changes every week or every other week. How often you need to do tank maintenance will depend on how quickly nitrates and phosphates accumulate, as well as how often alkalinity and other trace elements need to be replaced.

Sometimes parameters change quickly, like alkalinity, calcium, and magnesium, which will reflect on coral growth. These nutrients may need to be manually dosed.

Overall, stability is key. Though you should aim for keeping your parameters within these ranges, slightly imperfect yet consistent levels are much better than constantly changing ones. 

Increase Water Flow

Water flow is essential in your marine aquarium.

Dead spots can quickly lead to algae growth and excess nutrients. Most corals also rely on a constant influx and exchange of new and old nutrients carried by water currents.

It can take some time to get the water flow in your aquarium right, especially if dealing with corals. Oftentimes, reef hobbyists will need to add and remove equipment every once in a while to make up for denser coral growth. 

Those with fish-only (FO) setups may not need to worry about altering water flow but may need to make occasional small changes if the bioload suddenly increases.

It should also be noted that most aquarium equipment starts to lose power after significant usage. Regularly cleaning and testing equipment can help delay this, but eventually, those items will need to be replaced. 

Other Aquarium Equipment and Media

If you’re finding that you’re doing everything right but still have red slime algae growing, then you might need additional equipment.

Most large tanks run a protein skimmer. These machines remove organic waste before it can be processed by beneficial bacteria, meaning fewer excess nutrients are available in the water column for algae or cyanobacteria to use.

Some hobbyists have also found using different supplements to be effective, like ChemiClean Aquarium Treatment, which is specifically designed for tackling red slime algae. 

Though these methods will help control cyanobacteria, they won’t address the root of the problem, which is usually related to nutrients, water flow, lighting, or substrate. 

Adjust Lighting

The correct lighting can also be difficult to figure out. Many factors go into lighting besides picking the right fixture. Lighting spectrums, intensities, and photoperiods should also be considered. 

For the most part, lighting will be determined by the marine life you have in your aquarium. Fish-only systems need basic lighting while small polyp stony (SPS) coral-dominant systems need several top-of-the-line fixtures.

No matter what kind of lighting you have, it will take some trial and error to find the right spectrums, intensities, and photoperiods for your system.

These changes and settings can quickly lead to algae growth, including red slime algae. 

When it comes to altering lighting, it is strongly recommended to make changes over several weeks to not completely shock your aquarium. 


You might not think about it, but substrate can play a role in algae and cyanobacteria growth. In recent years, hobbyists have leaped using crushed coral to aragonite sand.

Though there is nothing wrong with crushed coral, its jagged characteristic makes it easier for leftover food and other waste and debris to get stuck. This can consequently lead to dead zones accumulating waste and growing algae.

Aragonite sand also catches a lot of waste and can be more difficult to vacuum than crushed coral. Inefficient tank maintenance can lead to algae growing on the substrate over time.

Some hobbyists have found that the best route is opting for a bare bottom setup. However, crushed coral and sand are completely feasible if tank maintenance is maintained and water flow is optimal. 

Final Thoughts

Dealing with any kind of algae in the aquarium setting can be hard and frustrating, but cyanobacteria is unique.

This bacteria has been around for billions of years and it’s about time we accept it as a natural part of our home ecosystems.

Instead of tearing down your tank, monitor water parameters, water flow, lighting, and any waste that accumulates on the substrate. Make small, gradual changes and you’ll find that your red slime algae outbreak resolves itself within no time!

If you have any questions about red slime algae, blue-green algae, or have had experience dealing with a particularly stubborn outbreak of cyanobacteria in your aquarium, don’t hesitate to leave a comment below! 

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