Aquarium Red Slime Algae Causes and Solutions

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

DEA - P. DONNINI / De Agostini Picture Library / Getty Images

Red slime algae is actually not a "true" algae at all; it is a bacteria that is technically known as cyanobacteria. Often considered to be the evolutionary link between bacteria and algae, cyanobacteria are among the oldest forms of life on earth and date back at least 3.5 billion years. These organisms produce oxygen as a byproduct of photosynthesis, and scientists believe that if it weren't for this microscopic organism, there would be no blue skies on Earth.

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

Cyanobacteria are often called blue-green algae. However, only about half of these organisms are actually blue-green in color. Most forms found in saltwater fall into ranges of other colors, 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

Excess growth of red slime algae typically is related to lighting and/or nutrients in the water. These are the two ingredients that all algae need to grow. When attempting a remedy, try each solution one at a time. Otherwise, when the problem subsides, there is no way to know where the problem was coming from and which solution worked to fix it. Start with one solution and see what results you get. If that one doesn't work, try another one, and so on, until the problem is resolved.

Red Slime Algae Solutions


The use of improper bulbs, lack of maintenance, and extended lighting hours are contributors that can lead to all sorts of algae problems. While these organisms do well in the 665 to 680 nanometer (nm) wavelength range, they are quite active between the 560 and 620 nm range as well.


  • 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.


Phosphates (PO4) and nitrates (NO3) are primary nutrient food sources for red and other slime algae. Phosphates are commonly introduced into aquariums through the use of unfiltered fresh tap water as well as many aquarium products that may contain higher-than-normal concentrations of PO4, such as sea salt mixes, activated carbon, KH buffers, foods, and many other sources. Also, for established reef tanks, the long-term use of Kalkwasser precipitates phosphates out of the water, and these phosphate-based compounds can settle on and in the live 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 excess DOCs to accumulate in an aquarium gives rise to nitrate (NO3) problems. However, nitrates can also be introduced in the same manner as phosphates, and because they are the final byproduct produced in the nitrogen cycling process, they can naturally build to high levels due to the lack of proper aquarium maintenance care. Another contributor to DOC/nitrate problems is when a new live rock is introduced, as the curing process can add nutrients when some organisms on the rock die off.


  • Practice good aquarium maintenance care: 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 wet/dry trickle-type filters, 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 live rock 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 powerhead or two, install a wavemaker or surge device, and/or increase the flow rate or efficiency of the filtration system, depending on the size of the aquarium.


Because slime algae consume nitrates, when aquarists perform nitrate tests, the readings may read as normal. If you were to remove the algae temporarily, you would likely see a rise in the nitrate levels in the aquarium, as the nitrates are undetectable when the algae is feeding on them.

Fighting Red Slime Algae With Additives

While cleaning up the tank and following proper maintenance care routines won't give immediate results, you can use one of a number of additives to remedy the problem quickly, within a day or two. However, be aware that many of these treatments solve only the symptom (the slime algae), not the underlying problem(s) causing the excess algae growth.

Given that cyanobacteria are a form of bacteria, many of the additives currently in use are antibiotics, which are medications that can weaken or totally wipe out the biological filter base of an aquarium. Be sure to use these types of treatments cautiously!