How To Get Rid Of Red Slime Algae

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Maintaining a clean and healthy tank is the pride of any fish-keeper – be they a professional or hobbyist. After all, the happiness of your fish and the aesthetic of your tank depend on your hard work.

So, how should you react if you find red algae in your fish tank? Does this mean you’ve done something wrong?

red slime algae

How dangerous is it to your fish? Can it be remedied?

Let’s take a look at what red slime algae really is, what dangers it poses to your tank, and how to get rid of it for good. By the end, you’ll be on your way to a healthier aquarium!

What is Red Slime Algae?

It may sound strange, but red slime algae isn’t actually algae at all! This is actually a type of bacteria known as red cyanobacteria.

Cyanobacteria are one of the oldest forms of life left on earth, with their origin dating back at least 3.5 billion years. Because of this, they are often considered to be the evolutionary link between today’s bacteria and algae.

While “cyanobacteria” literally means “blue-green bacteria”, only about half of this bacteria is actually blue-green in color. Most cyanobacteria found in saltwater can vary in color, from blackish green, to orange-yellow, copper-colored, deep purple, and even black.

If you notice a red slime (or other-colored slime) patch growing in your aquarium, be sure to eradicate it – quickly! It is an aggressive, relentless species that will quickly engulf your entire saltwater tank.

Dangers of Red Slime

While red slime algae is an unruly bacteria, universally hated by all saltwater aquarists, it is actually quite harmless. It is not dangerous to the fish living inside the infected tank.

So, what real problem does red algae in your saltwater tank pose (other than how gross it makes your tank look)?

It’s incredibly difficult to clean up and doesn’t leave without a fight.

red slime algae
By Peretz Partensky (Creative Commons Attribution-Share Alike 2.0 licence)

Causes of Red Slime Algae


The most common source of unwanted cyanobacteria growth is improper or poor lighting.

Most algae species, and especially red slime algae thrive in nanometers ranging between 640 to 680. Unfortunately, most corals require lighting between 400 to 700 nanometers to survive, so if you keep corals, this poses a very annoying problem.

Keep in mind that, as bulbs age, they tend to lose their intensity. This lower degree of lighting then allows for unsightly red slime algae growth.

Bulbs that measure below 10,000 Kelvin are the main culprit, so it is very important to change your set-up’s bulbs every 8 to 12 months, so as to prevent this inevitable energy dip.

Lack of Maintenance

Of course, neglecting your tank overall will create a whole slew of issues – red slime algae being the least of your worries.

Some people may change their aquarium’s water on an irregular basis, which can cause a build-up of excess nutrients that red slime algae will thrive on.

Excess Nutrients

Excess nutrients can stem from a wide variety of sources, including irregular water changes (as mentioned above), introducing a badly cured live rock, feeding your fish too much food, or even from a dead fish, which red slime algae can then use to grow.

In fact, aside from lighting, this is one of the most common causes of red slime algae growth.

Lack of Water Circulation

If you notice cyanobacteria growing in only certain parts of your aquarium, then those areas may have a lack of decent water circulation.

Waste or excess nutrients can quickly build up in low-flow, allowing red slime algae to develop and spread.

red cyanobacteria
By Derek Keats (Creative Commons Attribution-Share Alike 2.0 licence)

Not Enough Air

Fish and any plants in your tank need to breathe oxygen just as much as we do.

Unfortunately, red slime algae thrive on a lack of oxygen, so it is best to keep your tank properly aerated at all times.

Premature Live Rocks

As mentioned above, a live rock that is not entirely cured can easily become a breeding ground for red slime, rather than serving the purpose it’s meant for.

A poorly cured rock could kill off the organisms living on top of it, leaving extra nutrients floating around the tank that the algae uses to grow.

Chemically Imbalanced Water

Filling your tank with water that has too many nitrates or phosphates can mean rapid cyanobacteria growth in the future.

Dissolved Organic Compounds

Your tank can easily suffer from a sharp rise in nitrate and phosphate levels if you allow excess dissolved organic compounds to accumulate.

Phosphates can become especially prevalent since they are one of the final byproducts produced in the nitrogen cycling process. Thus, they build up to unsafe levels if you are negligent in your maintenance.

How to Get Rid of Red Slime Algae

Wash and Scrub

If you notice red slime algae growing in your tank, you’ll have to manually remove every bit yourself. Make sure you thoroughly wipe the glass and gently brush your hardier corals and rocks with a toothbrush.

red algae in fish tank
By Derek Keats (Creative Commons Attribution-Share Alike 2.0 licence)

Be Careful with Chemicals

If you choose to use any chemical products to clean your tank and substrate, be sure you read the directions on the bottle very carefully before applying it and to wash it off once you are done.

Water Changes

You should also increase your water changes to 30% a week, while using a high-quality water source, like distilled or RO/DI water. Continue this until you’re certain the algae will not grow again.

Change the Food

You should also stop feeding your fish any phosphate-rich foods like seasoned nori, liquified foods, gels, or low-quality fish meals.

This will help starve out the algae. While almost all food waste will eventually increase the water’s phosphate levels, cutting out more prominent sources may help.

You may also choose to use a phosban reactor or granulated ferric oxide to remove any excess phosphates in your tank as well.

Keep Up the Good Work

Even if you follow all these steps to a T, your tank may still be susceptible to another red slime attack.

Do not be discouraged if it does! This alga can be very persistent, and even the tiniest bit accidentally left inside can still spread across the tank.

So, how do you prevent it from coming back?

This video goes into more detail on how to get rid of red slime algae.

How to Prevent Red Cyanobacteria

Regular Water Changes

Frequent and regular water changes are especially important in the upkeep of your aquarium.

This will cycle out any waste and excess nutrients that the algae will thrive on, as well as balance out the minerals in your water.

Furthermore, use a reverse osmosis system as quality source water. This can help to prevent that dreaded excess build-up of unwanted chemicals.

Maintain High PH Levels

Be sure that you maintain the proper pH levels for your fish species.

Typically, you should aim for anywhere between 8.1 and 8.4 with buffers or with kalkwasser. High pH can discourage red slime algae from spreading.

Reduce Your Phosphate Levels

Be sure you replace your mechanical filter media often. If you find that your tank has become overrun with phosphates, it may help to use a phosphate-removing chemical filter media, until they reach optimum levels again.

Some aquarists have found that Tropic Marin Elimi-Phos proved to be especially helpful in removing excess pollutants like phosphate – since these are a favorite of the red slime to feed off.

You can buy this product in sachets, which can then be placed in your filter system to draw your tank’s extra pollutants. It is also an aluminum-based product, meaning its granules do not clump together and will instead remain porous, allowing water to flow freely through.

Phosphate levels should be kept at around 0.05 ppm or below.  However, keep in mind that phosphate can actually become “locked” in red slime algae, meaning that your phosphate levels may measure within an acceptable range, but your tank may still be overrun with algae.

As such, closely watch your aquarium’s phosphate levels to prevent any unwanted algae growth. A little ‘blip’ can still be a bad sign.

Since high phosphate levels can also be a result of the nitrogen cycling process, you should watch your nitrate levels, too. Your nitrates should always test below 10 ppm.

Adding chemical media like GFO or biopellets can help reduce these levels to more appropriate levels.

red cyanobacteria
By Derek Keats (Creative Commons Attribution-Share Alike 2.0 licence)

Clean Your Aquarium

In addition to regular water changes, you should also clean your substrate on a regular basis.

Using a siphon to remove any remaining organic waste and uneaten food will keep your tank fresh. As a bonus, it will help prevent the algae from latching onto anything, which it could then grow off of.

Do Not Overfeed Your Fish

It’s wise to only feed your tank inhabitants as much food as they can eat within a few minutes. This is to prevent the algae from spreading due to the extra food waste.

If your fish do not touch the leftover food bits after a few minutes, use a net to take these bits out before they float to the bottom. Otherwise, use your siphon to clean it all up.

Check the Ingredients of Your Fish’s Food

It’s also wise to check the foods you’re feeding your fish. Make sure you are not feeding them anything too high in phosphates or silicates, which can pollute your water and provide the algae fuel for growth.

If you like to feed your tank inhabitants frozen foods (like frozen mysid shrimp or brine shrimp, for example), be sure you rinse these foods in a strainer under warm tap water first.

Merely dumping the frozen food into the tank straight from the fridge will shock your fish, as the temperature change will impact the water quality.

The food may also contain chemicals or debris, gathered during the packaging process. A quick rinse will balance this out.

Increase Water Circulation In Low Flow (Or Dead) Areas

To increase water circulation in certain low-flow areas, you can add a powerhead or a wavemaker.

This may also provide your fish with a more natural habitat, as wild environments often have flowing water and currents.

Rearranging the rocks or other substrates in your aquarium will also allow for better water circulation.

Use a Protein Skimmer

A protein skimmer fills your tank’s water with tiny air bubbles that can help remove any dissolved organic compounds.

These compounds stick to the bubbles forming in the reaction chamber, and the resulting foam rises to the top, where it’s collected in a cup.

After that, you can easily dispose of the foam yourself.

Keep Fish That Will Eat the Algae

Some aquarium critters will eat just about anything, including red slime algae. As such, keeping species that will eat up this pest may be a fast solution.

Some of these species like to cling to the rocks and glass of your tank, searching for food, while other reef-safe algae eaters forage solely along the substrate.

Reef-safe hermit crab species, for instance, like to crawl over both. These crabs mostly survive on a diet of algae and waste that may be scattered throughout your tank. Considering their small size, you do not have to worry about them moving around your tank decorations in their frantic search for food.

On the other hand, some snail species and blennies prefer to spend their time on the rocks and glass in your tank. Both snails and blennies are herbivores that will attack any red slime algae and keep your tank clean.

Surgeonfish are another great herbivore species, operating as excellent glass and rock cleaners. Your corals will be safe around them, too, which is a great plus. Just be sure you keep them in a large tank, so they do not become cramped as they grow.

Here’s a video with more details on dealing with red cyanobacteria.


Red slime algae can be an annoying pest to deal with, but they are ultimately harmless to the fish in your tank.

Now that you know how to get rid of cyanobacteria and how to prevent it from growing again, hopefully, you can keep your tank as clear and healthy as it can be.

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