Causes and Treatments for Aquarium Red Slime Algae

Red algae (Peyssonnelia squamaria) and member of Serpulidae's family on a sponge

Red slime algae is really a bacterium known as cyanobacteria, and it is not a "genuine" algae at all. Cyanobacteria are among the earliest forms of life on Earth, dating back at least 3.5 billion years and are often regarded the evolutionary connection between bacteria and algae. As a result of photosynthesis, these creatures create oxygen, and experts think that there would be no blue sky on Earth if it weren't for this small organism.

What is Red Slime Algae?

Red slime algae isn't actually algae but a bacteria known as cyanobacteria, an evolutionary link between bacteria and algae dating back at least 3.5 billion years ago.

Growth of red slime algae is a common problem in saltwater aquariums, and it should be dealt with to ensure a healthy environment for your aquarium plants and animals.

Identifying Red Slime Algae

Blue-green algae are another name for cyanobacteria. Only roughly half of these species, however, are blue-green in hue. Most saltwater varieties are seen in a variety of hues, including:

  • Blackish green to blue-green
  • Orange-yellow to reddish-brown
  • Deep purple to fully black

Red slime algae start out as small patches but can quickly spread to form a mat that covers a large area.

Red Slime Algae Causes

Red slime algae development is usually influenced by illumination and/or nutrients in the water. All algae require these two components to thrive. Try each solution one at a time while attempting a treatment. Otherwise, when the problem goes away, there's no way of knowing what caused it or what remedy worked to solve it. Start with only one solution and observe what happens. If it doesn't work, try another until the issue is remedied.

Red Slime Algae Solutions


The use of ineffective bulbs, a lack of maintenance, and longer illumination hours are all factors that might contribute to algae growth. While these organisms thrive in the wavelength range of 665 to 680 nanometers (nm), they are also active between 560 and 620 nm.


  • Use only bulbs that are designed for aquarium use, with an appropriate total wattage for the tank.
  • Run the lights 8 to 9 hours a day, depending on the tank's lighting needs.
  • Try different types of bulbs to increase the intensity and optimize the spectral qualities of the light in the aquarium, particularly when it comes to any type of full-spectrum or color-enhancing tubes being used.


The principal nutritional food sources for red and other slime algae are phosphates (PO4) and (NO3). Phosphates are routinely introduced into aquariums via unfiltered fresh tap water and a variety of aquarium supplies that may include higher-than-normal quantities of PO4, including as sea salt blends, activated charcoal, KH buffers, meals, and a variety of other sources. Long-term usage of Kalkwasser removes phosphates from the water in established reef tanks, and these phosphate-based compounds can settle on and in the living rock and substrate.


  • Use RO/DI filtered make-up water and a high-quality sea salt mix.
  • Be aware of the elements contained in other common aquarium products you may be using.

Dissolved Organic Compounds (DOCs)

Allowing too much DOC to build up in an aquarium might lead to nitrate (NO3) issues. However, much like phosphates, nitrates may be supplied in the same way, and because they are the ultimate result of the nitrogen cycling process, they can naturally build up to excessive levels owing to poor aquarium management. When a fresh is installed, the curing process might contribute nutrients when certain organisms on the rock die off, contributing to DOC/nitrate issues.


  • Practice good : keep the substrate clean, cut back on feedings, rinse regularly, rejuvenate or change filtering or absorbing materials (filter flosses, cartridges, bio wheels, sponges, carbon), perform regular partial water changes.
  • Add a protein skimmer.
  • Use , for systems that have been running for some time. The bio media in them, especially bio balls, are real nitrate factories and therefore should be carefully rinsed and cleaned periodically.
  • Take the time to cure properly when adding it.
  • Add some tank-friendly algae/detritus-eating hermit crabs (left-handed or dwarf zebra hermit Crab), one or two true crabs, shrimps, or other good substrate-sifting tank janitors, or a fish (such as orange-spotted sleeper goby; Valenciennea puellaris).
  • Avoid adding new animals if your tank is still cycling. Also avoid any water changes and any major substrate or filter cleaning tasks, other than changing dirty pre-filtering materials or quickly siphoning stuff off the bottom, until the tank has completely finished cycling. Since red slime algae do not attach well, they can easily be peeled off and removed with light siphoning, and larger floating pieces can be removed with a net or turkey baster.

Carbon Dioxide (CO2)

Low water flow or movement throughout the aquarium produces carbon dioxide (CO2), which algae consume.


  • Add a or two, install a wavemaker or surge device, and/or increase the  or efficiency of the filtration system, depending on the size of the aquarium.


Because slime algae absorb nitrates, nitrate tests performed by aquarists may appear normal. Because nitrates are undetected when the algae is feeding on them, removing the algae for a short period of time will likely result in an increase in nitrate levels in the aquarium.

Fighting Red Slime Algae With Additives

While cleaning the tank and following correct maintenance care procedures won't solve the problem right away, you can use one of many additives to solve the problem in a day or two. However, keep in mind that many of these remedies simply address the symptom (slime algae), not the underlying issue(s) that are generating the excessive algae development.

Because cyanobacteria are bacteria, many of the current additions include antibiotics, which are pharmaceuticals that can weaken or completely wipe out an aquarium's biological filter foundation. Make careful to utilize these remedies with caution!