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How To Get Rid Of Red Slime Algae (Cyanobacteria)

Its tough to see your beautiful aquarium become over run with algae.

If you are anything like me, you would probably do just about anything to get your tank back to pristine conditions. In this guide, we will discuss all the strategies you can use to get rid of red slime algae (cyanobacteria) forever!

red slime algae

What is Red Slime Algae?

You may be surprised to find out that red slime algae is actually not an algae at all; it is a type of bacteria!

Red slime algae starts out in a small patch in your aquarium, but quickly spread outwards until it engulfs the entire tank. Anyone who has battled cyanobacteria before knows just how frustrating it can be.

It is relentless and exceptionally fast growing. When I battled it in my saltwater aquarium, it would grow back within a few hours. At one point, I considered tearing down my tank completely and starting over from scratch.

I came to find out, luckily, that there are a few ways to get rid of cyanobacteria for good without nuking your tank.

Though this guide, we will discuss all of ways to get rid of this pesky bacteria and restore your tank to pristine conditions.

Why Do I Have Red Slime Algae?

Pinpointing the reason for red slime algae in your tank is crucial to coming up with an effective solution. Over my years in the hobby, I have found that cyanobacteria usually propagates from two sources; lighting and nutrients.


Improper lighting is the most common cause of cyanobacteria in marine aquariums.

When I set up my first saltwater aquarium, I tried to take the cheap route and build my own T5 light fixture. Turns out, I messed some things up, used the wrong bulbs, and produced a haven for different algae.

Most algae, especially red slime algae, thrive in nanometers between 640-680. Most corals require lighting between 400-700 nanometers. This clearly bring forth a problem, because our beautiful corals feed off the same light as annoying algaes!

Bulbs that are below 10,000K are the most common cause for cyanobacteria outbreaks.

As bulbs age, they tend to lose their intensity and encourage algae grown. As a result, it is important to change your bulbs every 8-12 months.

Lack of Maintenance

Failing to maintain your reef tank will lead to tons of problems, algae being just one of them. Slacking on water changes, for example, causes nutrients to build up and throttle growth for cyanobacteria.

Keep up with water changes to help curb algae outbreaks.

Excess Nutrients

Excess nutrients can come from a variety of sources, such as the lack of water changes, badly cured live rock, or dead fish.

It is important to know that this can happen to anyone, and a cyanobacteria outbreak does not mean your a bad reef keeper.

Since red slime algae cannot possibly exist without proper nutrients, knowing how to get rid of nutrients will kill bacteria at the source. This is usually the most effective way to get rid of red slime algae for good!

How Do I Get Rid of Red Slime Algae?

After you have pinpointed the cause of your cyanobacteria outbreak, you are ready to start the fighting it!

If you think that the cause is a mixture of the sources above, don’t worry; this will work for you too. Here are a few steps you can take to finally get rid of red slime algae from your saltwater reef tank:

1. Cut Down on Your Lighting

One of the biggest causes of red slime algae is improper lighting. Make sure you are only using bulbs made for aquarium use. Anything else will surely cause algae outbreaks.

Don’t run your lights more than 7-9 hour a day. Too much light leads to explosive cyanobacteria growth.

If you do not have corals, you can also “blackout” the tank for 3-4 days. Keeping the light off for extended periods of time can get rid of red slime algae once and for all.

2. Keep Up With Regular Maintenance

Regularly changing the water in your reef tank is a good practice, whether you have algae or not.

While you may not see immediate results with this method, keeping a healthier reef will cut down on headaches immensely in the long run, including prevent algae outbreaks.

3. Eliminate Any Nitrate Factories

There was a time in reef keeping when bio balls and wet/dry filters were huge. Recent discoveries have proven that they usually do more harm than good, producing nitrates is mass quantity.

Algae, of course, feeds off these nitrates and blooms during spikes. If you can not eliminate these from your reef tank, make sure you rinse them regularly.

4. Cure Your Live Rock Properly

The biggest cause of red slime algae is excess nutrients. Improperly cured live rock will introduce tons of decaying material into your tank. This organic material causes nitrites, nitrates, and phosphates to spike, which red slime algae feeds on.

Eliminating these organic compounds from your tank will starve cyanobacteria at the source.

5. Add A Protein Skimmer

The number one reason for algae in saltwater aquariums is excess dissolved organics. Luckily, protein skimmers do a great job of removing these organic compounds before they can do any real harm.

Getting by without a protein skimmer is possible, but it might just be that last piece of equipment necessary to make your tank sparkling clean.

6. Get A Phosphate Reactor

Phosphate is cyanobacteria’s favorite food.

I set up a 40 gallon reef tank a few years back and red slime algae overtook it from the start. For months I tried everything, but I could not figure out the root of the problem.

Eventually I figured out that the base rock I bought was bad and leeching phosphates into my water, providing a haven for cyanobacteria. I purchased the Two Little Fishies ATLPBR150 GFO PhosBan Reactor 150 and immediately I noticed a difference. Check out our guide on the best phosphate reactors.

A few water changes and some new rock, my tank was cleaner than ever.

7. Increase Water Flow

Red slime algae can only grow strongly in places where water flow is weak. I would recommend placing a few Hydor Koralia powerheads around your tank to increase water flow and stunt the growth of cyanobacteria.

Koralias are powerful, cheap, and very reliable. In my opinion, there is no need to spend hundreds of dollars on powerhead right away. Koralias always do the trick for me.

Should I Use Chemicals To Get Rid Of Red Slime Algae?

Many blogs and websites suggest using chemicals to get rid of red slime algae.

While these do technically work, they usually end up doing more harm than good. Most of these chemicals are actually antibiotics (since red slime algae is actually a bacteria!), and tend to wipe out more than just the cyanobacteria.

Antibiotics will kill all the bacteria in your tank, good or bad. If you recall my article on cycling your saltwater tank, beneficial bacteria are responsible for keeping ammonia and nitrites in check. Without these beneficial bacteria, ammonia and nitrites can spike and wipe out your entire tank.

I think we would all agree that a little red slime algae is better than completely killing your livestock! For these reasons, I completely avoid chemicals when trying to get rid of red slime algae.

Final Notes

Red Slime Algae, also known as cyanobacteria, is extremely hard to eliminate for good. It can stem from a variety of sources, all of which require different solutions.

The best thing you can do is keep a healthy fish tank. By doing so, you will eliminate all sources of nutrients for the bacteria and your aquarium will look better than ever.

Though our complete guide, you should be able to get rid of red slime algae and enjoy all of the beauty that your tank has to offer. Enjoy!

Jason Roberts
About Jason Roberts
Jason is an aquarium fanatic that has been a fish hobbyist for almost three decades.

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