Coralline Algae and Cyanobacteria: What Are the Differences?

When operating your own saltwater ecosystem in a tank, algae become part and parcel to the experience.  Some algae are good and others are bad.

Coralline algae and cyanobacteria are two algae that can appear similar in color but only coralline are good enough to stay in the tank.  If you have cyanobacteria, you will have to blast it off with a turkey baste or other similar contraption. 

How do you identify coralline algae?

From the order of Corallinales, coralline algae are a type of red algae that comes from adding live rocks to an aquarium.  You can find coralline algae in any major ocean as far down as 1,000 feet.  It can withstand any environment so long as there’s a vast array of nutrients and plenty of sunlight.

Kinds of Algae

There are two different kinds of coralline algae:

  • Articulated: Also known as geniculate coralline, it appears tree-like with branching that’s flexible with noncalcified sections.
  • Unarticulated: Called nongeniculate coralline, it is the most common crust in a reef tank.  These slow-growing versions occur on live rock, shells, plastics, other algae, and coral skeletons.  Once mature, it provides a foundation for invertebrates to colonize and inhabit.


Although often red or pink, there are some coralline algae that come in an array of vibrant colors like green, white, yellow, gray, blue or even purple. 


Crustose coralline algae create limestone deposits that form a tough outer crust of aragonite.  This develops over any part of a reef that continues to build on top of itself, making for a safe and strong space for the corals and fish that live within them

Purpose of Coralline Algae

You can think of coralline algae like a cement that prevents newly developed corals from coming apart or breaking off the reef.  This is important in the wild, where storms and other oceanic activity can be violent.

When you see this growing, it’s a sign of a well-matured marine tank that has joined the same natural processes as the ocean. No matter how much of it grows in your aquarium, it will not pose a danger to inhabiting plants or fish.

Coralline’s Importance

This kind of algae is perfect for many saltwater fish, acting as a renewable and abundant food source for them.  They provide minerals, nutrients, and other healthy substances that are difficult for you to feed fish. 

It also provides living quarters for invertebrates with tiny spaces for them to live and hide in.  This provides even more nutrients for fish and other aquatic life that has invertebrates as part of their regular diet.  So, encouraging the growth of coralline is crucial to the health of your tank.

Will coralline algae grow by itself?

No, coralline algae will not grow by itself; you have to introduce it into an aquarium and encourage its development.  Adding live rock is the most common way to introduce it, but so will snails or hermit crabs that have it hitchhiking on their shells.

Once introduced, you have to ensure the perfect water parameters for the algae to thrive:

Stay on top of stable water temperature and chemistry. Smaller, frequent water changes are better than large, scarce changes.  Doing this mitigates shock to the tank.

  • Ensure nitrates stay below five ppm
  • Phosphates should be below 0.25 ppm
  • Maintain a pH between 8.1 and 8.3.  You can encourage and control it with a KH level of 2.8 meg/L
  • The water should be rich in magnesium and calcium to encourage coralline algae’s growth
  • Lighting is the most important aspect of having coralline, which helps them produce vibrant colors.  If you cannot give the tank natural access to daily sunlight, reproduce it with strong LED lights.

What does cyanobacteria look like?

Cyanobacteria, also known as Red Slime Algae, is the evolutionary connection between algae and bacteria, it’s not an actual algae.  It’s one of the earth’s oldest life forms, dating back to over three billion years ago. 

Most aquarists consider this a nuisance but it does serve an important purpose because it raises oxygen levels in the atmosphere.

It’s deep red, burgundy color with a stringy and slimy texture and appearance is how you identify its existence in your tank.  It sometimes will have bubbles, producing the oxygen byproduct.  It begins as a tiny patch on live rock or sand but grows quickly and overtakes your aquarium.  

Will cyanobacteria go away by itself?

No, cyanobacteria will not go away by itself.  But the good news is that it isn’t difficult to remove, so long as you tackle the problem early.  Do the following three things to ensure all bacteria clears from the tank:

  1. First, increase your tank’s water flow right to the area where the cyanobacteria are growing.  Any amount of water pressure, like from a turkey baste, will easily remove it from any surface it’s overtaking.
  2. Because cyanobacteria require nitrates and phosphates, eliminate or reduce your fish feedings so that you don’t give the algae fuel for growth.
  3. Reduce the amount of the tank’s sunlight exposure to truncate the cyanobacteria’s ability to continue its development.

You can do a combination of these or one at a time to pinpoint the cause.  But it is advisable to do all three to ensure the bacteria goes away and never returns.

Will cyanobacteria kill corals?

Yes, cyanobacteria will kill corals as well as fish, live rocks, and invertebrates.  It’s very toxic to most aquatic life, so it’s important to rid your tank of it the moment you notice it.

Coralline Algae vs. Cyanobacteria

Even though both coralline and cyanobacteria algae look the same in color, they do not serve the same purpose in your tank.  You want coralline for your saltwater aquatic life to thrive and be happy. 

Aside from cyanobacteria’s important role in the ocean, it’s not good for your fish, corals, and other aquatic life.  Understanding the difference between the two will determine a beautiful, thriving tank or a dead one.


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