What Causes Cyanobacteria In Reef Tanks?

In this article, I will explain the causes of cyanobacteria plus how to remove it and prevent it from happening in the first place.

Cyanobacteria is also known as Red slime algae and is one of the most common problems that hobbyists face. 

This bacteria can be a huge nuisance covering your rocks and sand with a deep red looking slime and can be quite invasive. 

You don’t want to let cyanobacteria get out of control. It should be addressed and removed right away. 

But don’t worry if you see it in your tank. It is a common occurrence mainly in new saltwater thanks but many experienced hobbyists deal with it as well.

Providing you recognize it and don’t let it take over your tank, it shouldn’t cause too many concerns.

It’s important to understand what causes cyanobacteria in the first place in order to prevent it from happening in your tank. 

Image: Marine Depot

Causes of cyanobacteria

There are many things that can contribute to this bacteria forming in your tank.

The main causes are:

  • Poor lighting
  • Poor water movement
  • Overfeeding
  • Tap water or RODI water with over 0 TDS
  • Poor protein skimming

All of these items are things that you can address and fix quite easily.

causes of cyanobacteria
Causes of cyanobacteria

Poor lighting

A big contributor to bacteria growth is poor lighting. If you use T5 lighting on your tank, the bulbs will have a maximum number of hours or lifespan on them and they will need to be replaced over time.

New bulbs will have around 2 times the PAR output as older bulbs. Usually, around the 8 months to 1-year mark, it’s time to start changing the bulbs.

If you notice cyanobacteria starting to grow in your tank, check how long the bulbs have been in use. An older light bulb will still produce light but the quality of light will be poor and the spectrum change will contribute to algae growth.

If you use LED’s for lighting your tank, monitor them over time and when the diodes start to dim or shift colors, it is time for a change. LED’s will last longer than T5’s but both will become less effective over time and will need replacement. 

Poor water movement

Low water circulation in your tank is another main cause of cyanobacteria. Proper water movement is crucial in reef tanks for many reasons and if you have a low current in your tank, the chances of getting cyanobacteria are higher.

A good indication of poor water flow is if you notice cyanobacteria forming in certain locations in the tank. Low flow areas like corners and behind rocks can be considered dead zones where the water is stagnant causing a build up of detritus and the poor aerated section is a breeding ground for bacteria. 

Image: Marine Depot


Overfeeding your aquarium is one of the most common mistakes hobbyists make. Uneaten foods that are not removed will cause poor water quality leading to high phosphate and nitrate concerns.

Overfeeding can be a cause of cyanobacteria. An interesting fact about cyanobacteria is that it tends to absorb and lock in phosphate so it’s quite common to have Red slime forming in your tank while the phosphate levels test within limits.

Keeping both nitrate and phosphate levels low and within the normal range is crucial to prevent cyanobacteria from forming. 

Tap water or RODI water with over 0 TDS

Using tap water for your saltwater tank is never a good idea. It is also not a good idea to use RODI water if the TDS is above 0.

Cyanobacteria will normally start to form when we ease up on regular maintenance and water change routines. If your RODI filter is not producing 0 TDS, it’s time for a filter or resin change.

Water changes are crucial to removing excess nutrients and keeping the water parameters within limits. Without good quality RODI water and a regular maintenance routine, expect to see cyanobacteria in your tank.

Poor protein skimming

A good quality protein skimmer that is properly rated for your tank size will be very effective in maintaining overall water quality and tank health.

One of the many benefits of using a protein skimmer is to provide your tank with increased oxygen. Lack of oxygen in your tank is a cause of cyanobacteria and using a protein skimmer will provide the oxygen your tank needs. 

How to get rid of cyanobacteria

If you have a breakout of cyanobacteria in your tank, you will want to take action to remove it. Luckily there are a few ways to take care of it.

Keep in mind that even though you are able to remove the cyanobacteria from your tank, you need to address the reasons why it occurred in the first place to prevent it from returning.

Most times the removal procedures are also great preventatives. 

Increase water changes

As mentioned above, using a poor water source can cause cyanobacteria. A great way to help remove it is to increase the amount of water changes with RODI water that has 0 TDS.

If you are normally performing water changes every 2 weeks, you may need to double the frequency and do it every week. Not only should you increase the number of water changes, but the volume of water changed should be increased. 

Manually remove it

While you are performing a water change, this is a good time to manually remove as much of the algae as possible. Using your water change hose, vacuum up as much as possible.

You can use an algae brush or a toothbrush to scrub the rocks and let it be sucked up the hose at the same time. Siphon out as much as possible during your water change. 

Use a chemical treatment

One of the best products on the market for treating Red slime algae is Chemiclean. This stuff is amazing and it works great. If you have exhausted all the non-chemical treatments and the cyanobacteria is getting out of control, try Chemiclean. 

When using chemiclean, it’s important to follow the instructions exactly. When used properly, Chemiclean is a safe and effective product that will remove Red slime algae from your tank.  

What eats red algae in saltwater tanks?

There are certain members of a clean up crew that are known to eat Red slime algae. Invertebrates such as Trochus and Cerith snails are good options along with urchins.

Keep in mind that although members of a cleanup crew are valuable and do a great job eating algae, they do not address the underlying issue of why the bacteria is there to begin with. 


Cyanobacteria can be a real nuisance and easily take over your tank. The best way to address it is to understand the causes and take care of those issues first. Don’t put a bandaid on it and only temporarily fix the problem where it will keep coming back. Fix the cause, fix the problem.