What Is Red Slime Algae? Easy Ways To Battle It

You have just got home from work and your beautifully pristine new reef aquarium now has this red slime all over it and your panicking! Do not worry! We all go through this and it is just part of your aquarium maturing.

Red Slime Algae is not actually an algae, but rather a bacteria called Cyanobacteria that multiply into large numbers when their food source Nitrate & Phosphate are abundant in an aquarium. The bacteria forms into a film covering sand & corals & if left untreated can starve them of light and oxygen.

Pretty much every new aquarium will go through this stage at some point in its early years. Some owners experience it just once, and others may see it pop back a few times.

It is easy to deal with you just have to understand why it’s happening. Keep reading to find out all you need to know about Cyanobacteria…

What Is Red Slime Algae?

As mentioned, Red Slime Algae is the nickname given to Cyanobacteria because that is exactly what it looks like in our aquarium.

Cyanobacteria are a strain of bacteria that live in water and photosynthesize like all plants. They are simple organisms but can link together in large chains to make them visible to us.

A Typical Cyano Outbreak

Another type of Cyanobacteria you may be familiar with is the Blue-Green Algae that can take over lakes and ponds when conditions are right. Although its the same bacteria, its a different species (I hope that is the correct term to use – Sorry Biologists If I’m way off the mark!) to the type we get in our home aquarium.

As with many things that grow in our aquarium the Cyanobacteria does not just appear as red slime. Sometimes it can be dark red, purple, dark brown and even black, it really all depends on the lighting you have.

Sometimes it can grow over your rocks in a thick, smothering film and other times it can appear as a dust growing everywhere. No matter which type you have, it needs to be dealt with to prevent it from getting worse.

Why Does Red Slime Algae Appear In An Aquarium?

The cells of Cyanobacteria are constantly present in our aquariums and most will lie undetected until conditions are just right for them to explode in reproduction and you get an outbreak.

Being a plant-based bacteria it needs several food sources for it to multiply. In our aquariums those are usually the following:

  • Detritus – Allowing uneaten food and fish waste to accumulate in your aquarium is going to provide a huge food source for the bacteria to feed on.
  • Phosphates – All plant-based organisms thrive when there are high levels of phosphates.
  • Nitrates – Just like phosphates, plant-based organisms love this stuff!

These 3 food sources can be found in abundance in a saltwater aquarium if the owner is unaware of them or a lack of regular maintenance has been allowed to creep in and given the parameters some time to rise.

Are There Ways To Prevent Red Slime Algae In An Aquarium?

Cyanobacteria is going to happen, and for most of us, its usually just after the aquarium has finished its Nitrogen Cycle. The Cyanobacteria cells will reach a point where the aquarium is just right to allow the bacteria to bloom.

If you let the bacteria run its course it will consume all of its food sources and die out – Providing you don’t go adding more detritus, phosphate or nitrates into your aquarium.

This can scare many newcomers to the hobby and they are always hasty to find a chemical solution to treat this mess.

Prevention Is Way Easier Than Curing!

But, before you go trying a chemical solution, here are a few other options to look at to try and pinpoint the cause first:-

A lack of flow can help bring on the bacteria bloom – Re-aligning your powerheads and wavemakers towards the sand bed can help.
If you have a lot of rock in your aquarium you may need additional powerheads to prevent the water from stagnating in certain areas.

How long is your lighting period? If you do not have corals yet your aquarium lights do not need to be on from 8am to 11pm. Reduce the lighting period to just the evening – This will reduce the photosynthesizing period of the bacteria.

What is your Source Water? – This is where the water you put into your aquarium comes from. Water from the tap and treated with a de-chlorinator like Prime, water from the local grocery store or sometimes water from your local fish store can be high in nitrates, phosphates, heavy metals, and other organic pollutants if their water filters are not replaced regularly. The only way to ensure the highest quality water for your aquarium is to install an RO/DI water filter in your home.

Dirty Source Water!

Live/Dry Rock leaching phosphates is another source of food for the bacteria – If this is the case it is too late to do anything about it now, just allow the rock to leach out the phosphate over time, but be aware your Cyano may stick around for a while. Buying your Live or Dry rock from reputable sources is a great way to prevent this.

Prevent Detritus from settling in your aquarium. Each week I get a turkey baster and ‘Blast’ all my rock. You will be surprised how much junk comes out of all those holes, cracks and pockets. Do this just before you do a water change each week and it will really help. Good flow will help to prevent it settle in the first place!

Overfeeding is a sure-fire way to give Cyanobacteria ample food! We are all guilty of it, especially when we are new to this hobby! Your fish will survive weeks without food. Feed them sparingly!

Overstocking your aquarium and adding too many fish too quickly will also allow your nitrates to rise because the Nitrifying Bacteria is not in enough numbers to handle the waste. Slow and steady wins the race when stocking. Keeping your fish to a minimum in the first year will help too!

What Are The Ways To Eliminate Red Slime Algae In Your Aquarium?

After you have looked into the possible causes above as to why your Cyanobacteria outbreak happened, the first thing I would do is to take measures to prevent it from happening again in the future.

Go through each step above and see if you can improve on your current setup, routine, or processes to help keep the 3 food sources to a minimum. By doing this you will also help to prevent other nuisance algae outbreaks too!

Before jumping into a chemical Cyano removal product to break it down here are a few tips to do first to help reduce it:

  1. Use your gravel vacuum to suck up as much as you can. You may have to do this every day, but the more you remove, the fewer bacteria are in the water to continue multiplying.
  2. Run some Activated Carbon or Chemi-Pure Elite (Links to Amazon.com) in a mesh bag in your sump or filter area to help consume any organic waste compounds, nitrates, and phosphates.
Chemi-Pure Elite
Chemi-Pure Elite

If after several weeks your daily battle against Cyano is still continuing then it might be time to look a chemical method of elimination.

By far the three most popular treatments for Cyanobacteria available are:-

  1. Chemiclean by Boyd Enterprises
  2. Red Slime Remover by Ultralife
  3. Red Cyano Rx & Green Cyano Rx by Blue Life USA

All links Above to Amazon.com


The only brand I personally have tried and had success with has been the Chemiclean by Boyd Enterprises. Some Cyanobacteria removal products use antibiotics to kill the Cyanobacteria cells, but this can also kill your helpful Nitrifying Bacteria cells too!

Chemiclean is not an antibiotic-based treatment and will not harm other cells in your aquarium. This, along with the track record and great reviews were the reasons I selected to use it – And It worked very well!

Be sure to read the directions carefully and follow them to the letter as using Chemiclean will send your Protein Skimmer nuts and cause it to overflow and could cause a drop in your oxygen level in the aquarium water. A simple air pump and airstone will help prevent this drop.

Vibrant Aquarium Cleaner

Many aquarists seem to be getting very good results from a fairly new product called Vibrant Aquarium Cleaner by Underwater Creations.

Vibrant uses multiple strains of bacteria to attack the algae from different angles. One of the bacteria strains targets the Nitrates and Phosphates that feed your algae and turn them into biomass to be removed via your Protein Skimmer or water changes.

This product is also designed to work on any problem algae, according to the manufacturer.

You can read the forum thread HERE on Reef2Reef where Jeff the owner comes on and joins in the conversation regarding it! – Cool!

“Vibrant is a true beast and we have not yet come across a algae that Vibrant can not beat out. Below I will list a general timeline of how fast Vibrant works on frequent algae strains that cause issues in reef aquariums”.

Cloudy/hazy Water– 1 dose
Diatoms – 1-2 doses
Cyanobacteria – (Yes, it will outcompete another bacteria) 1-5 doses
Dinoflagellates – 2-5 doses
Bubble algae – 3-8 doses
Hair Algae – 3-5 doses (depending on species of hair and how bad the infestation is)
Turf Algae – 8-20 doses ( again, depending on species and how bad the infestation is)
Bryopsis – 6-30 doses ( again, depending on species and how bad the infestation is)

Jeff Jacobson
Owner – Underwater Creations, Inc.

TBR Recommends

Vibrant Aquarium Cleaner comes in 8oz and 16oz bottles.

This great product is getting fantastic results and the many owners in the forums are confirming this!

Cyano Outbreak In A Mature Aquarium

So you have been through all the teething troubles of a new aquarium and are several years into your beautiful reef aquarium and Cyano rears its ugly head again. Why?

Here are a few reasons that life could have got in the way and given this silent stalker a chance to say hello again!…

Light Bulbs Need Replacing – Certain bulbs like Metal Halide and T5 bulbs have a useful lifespan. Anywhere from 8-18 months depending on the manufacturer. After this period their color spectrum can change and their intensity can fade. An old bulb can be just enough to set off a Cyano outbreak.

Coral Growth Restricting Flow – Something every Reefer aspires to! A glass box full to brimming with beautiful coral, but as your corals grow they impede flow. You may need to thin out the heard or install more powerheads/wavemakers to prevent any flow dead spots.

A Stunning But Full Aquarium

RO/DI Filters Need Replacing – You may have been diligent on keeping up with replacing your DI Resin and sediment & carbon filters, but the useful life of most RO membranes is around 2 years. The expiration of this part of your RO/DI filter is usually a good cause for a Cyano bloom.

A Buildup of Detritus – Has your coral growth and lack of flow prevented your detritus from remaining waterborne and removed by the filter media? Turkey basting, sandbed vacuuming, and good flow will help prevent it.

Overfeeding/Overstocking – Have you been too busy buying or feeding and overloaded your biological filter? Have you been on vacation and your tank sitter/autofeeder been giving large portions?

Lack of Maintenance – Has something just happened in your life that made you take a break from the regular maintenance routine? If your aquarium is used to weekly maintenance and its been a month, then this could be the cause.

Irregular Water Change – This ties into the one above. Life can get busy, things happen we cannot foresee but a regular testing, waterchange and maintenance routine will cure 90% of all the problems you will face with your aquarium!

To Finish

Red Slime Algae or Cyano is going to happen at some point. It can be a pain in the ass but if your aquarium is young, let it run its course and then take steps to control its food source.

By making a few simple changes listed in this article you can prevent its return. If it does come back, find the reason why, fix it then the easy treatments will soon have this ugly intruder put back to where it came from.

Further Reading

To find out more in-depth information about Cyanobacteria click HERE to go to the:

University of California Museum of Paleontology – UCMP article: “Introduction to the Cyanobacteria

You may find the following articles helpful as some of these aspects were mentioned above:


Hi, I'm Richard and I have been an avid aquarist for over 30 years with a passion for Saltwater Aquariums. I love to pass on my knowledge to help others get the same amount a pleasure out of this hobby as I do. View my About Me page to find out more about me & my mixed reef aquarium.

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