Red Slime Algae: What Is It And How Do You Get Rid Of It?

If you’re anything like most aquarium owners, you care deeply about your fish. You are meticulous about your reef tank and want your aquarium in tiptop condition. That’s exactly why red slime algae are such unwelcome visitors in your aquarium’s ecosystem. Red algae can blanket tank walls, live rocks, and even invertebrates such as clams and corals. They spread like wildfire, if wildfire could spread underwater, that is.

If you have red slime algae in your aquarium, there may be a simple explanation. To restore your aquarium to its original pristine state, you’ll need to understand a few things about what red slime algae is, why they grow, and how to rid your aquarium of this bacteria.

Don’t be discouraged if you find your tank coated with these invasive organisms. It’s a common problem that all aquarium owners face at one point or another. Here, we’ll dive deep into this dreaded aquatic menace and learn how to keep it at bay.

What Exactly Is Red Slime Algae?

Also called cyanobacteria, red slime algae are photosynthetic organisms. Contrary to the name, red slime algae are bacteria with a red pigment.

They are extremely fast-growing, and before you know it, a small amount can engulf your entire aquarium. They are one of the oldest known organisms, and they sure are resilient; red slime can grow back in just a couple of hours!

What Are The Causes of Red Slime Algae?

There may be a couple of culprits behind the algae bloom in your aquarium. Once you have identified the cause, you’ll be able to get rid of the aquatic nuisance.

Here are the main reasons red slime algae take over a tank:


Believe it or not, the cause behind your overrun aquarium and horrible profile picture may be the same: bad lighting! Bulbs below 10,000 kilowatts provide ideal conditions for cyanobacteria to grow. Don’t try to save by using cheap bulbs. You’ll end up paying for it later.

Keep in mind that bulbs begin to dim after 8-12 months, creating an environment that encourages algae growth. Replace bulbs frequently to avoid creating a haven for cyanobacteria.

Unclean Tank

Every aquarium enthusiast knows that regularly cleaning your tank is a must. Slacking on this can lead to lots of problems in the long run, red slime algae included. Not changing the water in your tank frequently enough can create the perfect habitat for cyanobacteria.

Excess Nutrients

There are several reasons why excess nutrients might be building up in your tank and causing red slime algae to grow.

The most common causes are:

  • A dead fish
  • An improperly cured live rock
  • Unchanged water

Red slime algae cannot live on live rock without the proper nutrients. Therefore, if you get rid of the nutrients, you’ll get rid of the red slime as well. Let’s take a closer look at exactly how to get rid of red slime in a saltwater aquarium.

How Do I Get Rid of Cyanobacteria?

get rid of red slime on live rok
How to Get Rid of Red Slime!

Now that you understand what red algae are and why they might be in your tank, let’s explore how to get rid of them. If you aren’t sure what prompted the bacteria growth or think it might be a combination of things, don’t worry. All of these tips will help protect your aquarium against red slime algae.

Often the best way to eliminate cyanobacteria for good is a combination of removal and prevention.

Reduce Your Lighting

Cyanobacteria needs a light source to grow as this bacteria is a photosynthetic organism. Try reducing your light level slightly and only leaving the light on for 7-9 hours a day. This will prevent red slime algae from growing rapidly. Just don’t forget about any other photosynthesizes in your tank, such as coral!

Regular Maintenance

The importance of regular aquarium maintenance really can’t be overstated. Regular water changes (at least once a week) prevent nutrients that sustain cyanobacteria from building up in your tank. If you find that you don’t have time for weekly water changes, replace a greater volume of water each time you clean.

If you have cyanobacteria in your tank, you can use a toothbrush to remove the residue from rocks and surfaces. Vacuum up any floating bits dislodged during your brushing.

Be Wary of Nitrates

Beware of any objects in your aquarium that may be producing nitrates en masse, such as wet/dry filters or bio-balls. Nitrates can also build up due to infrequent water changes or a poor water source. Nitrates are one of the leading causes of red slime algae growth and can even harm your fish if levels get too high.

The water you’re using could also be full of nitrates. Non-purified tap water is often teeming with nitrates and phosphates. Consider using an RO/DI water filtration system to separate any dissolved nutrients that red slime algae feed on.

Make Sure Live Rocks are Cured Properly

Live rocks that aren’t cured properly are like assembly lines for decaying materials such as nitrates, phosphates, and nitrites. These excess nutrients will build up, and red slime algae will feed on them. Refrain from adding any live rocks that aren’t fully cured to your tank unless you want a red slime algae explosion.

Use a Protein Skimmer

If you’re past the prevention stage and have an aquarium full of cyanobacteria, you’ll need to know how to remove red algae from a saltwater tank. Protein skimmers are perfect for the job. They remove the organic compounds that red slime algae feed on before growth gets out of hand.

Protein skimmers also fill the water in your tank with air bubbles. Organic compounds that would normally build up and allow cyanobacteria to grow sticks to these air bubbles instead. Foam is then formed and transferred to a collection cup, keeping problematic organic compounds out of your tank.

Invest In a Phosphate Reactor

The only thing cyanobacteria love more than nitrates is phosphates. Where phosphates are, red slime algae is sure to follow. A phosphate reactor gives you peace of mind, eliminating excess phosphates that build up in your tank over time.


Increase Your Water Flow

In areas where water flow is weak, cyanobacteria can thrive. Placing powerheads throughout your tank will increase water flow, preventing the growth of red slime algae. Powerheads tend to be relatively inexpensive and make a world of difference.

What About Chemical Tools?

Some aquarium enthusiasts sites might recommend that you use chemicals to fix your red slime algae problem. Although it may seem like a miracle cure at first, the long-term results are less than desirable. Most red slime fighting chemicals are antibiotics (to wipe out the cyanobacteria) and end up killing more than the red slime.

Your tank also has good bacteria in it, bacteria that help keep the ammonia and nitrates levels of your tank in check. If these friendly bacteria are wiped out, your aquarium will be in real trouble.

Although red slime algae are unsightly and annoying, this bacteria is not nearly as harmful to your fish as eliminating all the beneficial bacteria from their habitat. It’s best to avoid chemical solutions when it comes to curbing red slime algae.

Antibiotics are also only a band-aid and will not address the underlying cause of the red algae slime. It’s best to eliminate the problem at the source, which is likely an issue with lighting, cleanliness, or nutrients buildup.

Will Red Algae Ever Go Away For Good?

The battle against red slime algae is an ongoing fight for reef tank aquarium owners. This bacteria spawns for a variety of reasons and grows rapidly once present. However, it has its weaknesses.

Now that you know how to get rid of red slime and how to prevent it, your reef tank will be cleaner, and your fish friends will thank you.

Kelly Stanley