Red Algae In A Saltwater Aquarium &; 4 Smart Ways To Control Them

Problems with red algae in a saltwater aquarium? Keep reading and learn how to prevent and cure their occurrence in your aquarium.

 Are you ready for one interesting fact to start with? Namely, although they are so-called, red algae are not algae at all, but a type of bacteria, technically known as cyanobacteria. They are considered a common, evolutionary link between bacteria and algae.

Did you know that cyanobacteria are among the oldest forms of life on Earth? Namely, they date back at least 3.5 billion years. Amazing isn’t it? Stay with us and find out even more details about red algae in a saltwater aquarium.

What Are Red Algae?

Red algae belong to the Rhodophyta division, which includes about 6,000 species of predominantly marine algae, often found attached to other shore plants. As for color, they usually appear in red or blue. Their color is the result of the masking of chlorophyll by phycobilin pigments.

Red algae start in a small patch and then spread very quickly to other parts of the aquarium. They are relentless, grow and expand fairly quickly, and are able to occupy an entire tank in a matter of hours. Continue reading to learn more about red algae in a saltwater aquarium.

 Is red algae harmful to saltwater fish?

Where Do Red Algae In A Saltwater Aquarium Come From?

Determining the cause of red algae in your tank is key to finding an effective solution. Their overgrowth is usually associated with lighting and/or nutrients in the water. So, these are the two ingredients they need to progress. When attempting a remedy, try one method at a time. Only then will you be sure what helped you and what didn’t.

Cheat-Sheet To Solve This Problem


The use of improper bulbs, lack of maintenance, and extended lighting hours are just some of the reasons for the expansion of red algae in a saltwater aquarium. Therefore, it is recommended to use only those bulbs designed for aquarium use, with an appropriate total wattage for the tank.

You can even try different types of bulbs in order to increase the intensity and optimize the spectral qualities of the light in the fish tank. The lights need to be kept on for an average of 8 to 9 hours a day; however, it all depends on the tank’s lighting needs.

Read more about The Best Aquarium Light To Prevent Algae.

 red algae in a saltwater aquarium


Phosphates (PO4), as well as nitrates (NO3), are primary sources of nutrients for red algae. They are usually introduced into your aquarium using unfiltered fresh tap water, but also using a variety of products such as sea salt mixes, activated carbon, KH buffers, foods, and many other sources.

Keep in mind that long-term use of Kalkwasser precipitates phosphates out of the water, which ultimately results in the deposition of these phosphate-based compounds on and in living rock and substrate.

As a solution, it is proposed to use RO / DI filtered make-up water and a high-quality sea salt mixture.

Dissolved Organic Compounds

Accumulation of too many dissolved organic compounds affects nitrate (NO3) rise problems. In addition, another contribution to the problem of dissolved organic compounds/nitrates is the introduction of new living rock. Namely, there is a curing process that can add nutrients when some organisms on the rock die.

As a solution, the following are suggested: maintaining hygiene and regular cleaning, using wet/dry trickle-type filters, and adding tank-friendly algae / detritus-eating hermit crabs. In addition, it is recommended to avoid adding new animals if your tank is still cycling, as well as any sudden water, substrate, or filter changes, other than changing dirty pre-filtering materials or quickly siphoning, until the tank has completely finished cycling.

Carbon Dioxide 

If your aquarium tank has low water flow or movement, this will in most cases result in an increase in carbon dioxide (CO2), which is eventually consumed by algae.

As a solution for red algae in a saltwater aquarium, it is suggested to add a powerhead, install a wavemaker or surge device, and/or increase the flow rate or efficiency of the filtration system.

How To Fight Red Algae In A Saltwater Aquarium With Additives?

Cleaning up the tank and following proper maintenance care may not bear fruit immediately, but additives could remedy the problem quickly, within just a few days. However, keep in mind that most of these treatments will only solve the symptom, but not the underlying problem(s) that actually cause the algae to overgrowth.

Given that cyanobacteria are a form of bacteria, most drugs are actually antibiotics, which you should use with caution and according to the instructions because they can weaken or even wipe out the biological filter base of the fish tank.

In Conclusion

Red algae, also known as cyanobacteria, are extremely difficult to eliminate once found in an aquarium. Their spread is caused by different sources, each of which requires different solutions. Keep in mind that maintaining a healthy aquarium is a good precaution to avoid the emergence and spread of their population.

Have you ever encountered red algae in a saltwater aquarium and how did you solve that problem? Write us your experiences in the comments.

Take a look at this article as well.

 What eats red algae in saltwater tank?


Is Red Algae Harmful To Saltwater Fish?

Although unpleasant to the eye, red algae are rarely harmful to saltwater fish. In any case, it is better to remove than to leave them, because they have the ability to expand and occupy the entire tank.  

What Eats Red Algae In A Saltwater Tank?

Here are some of the most popular saltwater algae eaters: Hermit Crabs (Red Tip, Yellow Tip, Zebra Tip, Electric Blue, Electric Orange, Halloween, and Scarlet Reef). Snails (Chestnut Cowries, Margarita, Nerite, Mexican Turbo, Zebra Turbo). Blennies (Black Combtooth, Short Bodied, Highfin, Starry, Tail Spot, Two Spot). Tangs and Surgeonfish (Pacific Blue, Red Sea Sailfin, Goldrim, Orangebar, Powder Blue).

What Causes Red Slime In A Saltwater Tank?

Phosphates, nitrates, and other dissolved organic compounds are considered to be the most common causes of red slime in a saltwater tank. 

Where Can Red Algae Be Found?

Red algae are not established in just one place, in fact, they can be found around the world, from polar waters to the tropics, and are commonly found in tide pools and in coral reefs as well.