Red Slime Algae (Cyanobacteria) Description

What Is Red Slime Algae?

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 but classified as cyanobacteria. Often considered to be the evolutionary link between bacteria and algae, cyanobacteria are one of 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.

Commonly referred to as "red slime" algae, the name cyanobacteria literally means "blue-green" algae. Despite the naming, only about half of these organisms are actually blue-green in color. Most forms found in saltwater are other colors, ranging from blackish green to blue-green, from orange-yellow to reddish-brown, and often appear deep purple to fully black in color. Starting out as small patches, it spreads out from there as a mat of sheeting covering.

What Makes Slime Algae Grow and Solutions for Eliminating This Problem

We suggest that you don't try to put all of these solutions into action at one time because if you do when the problem subsides you'll never really know where the problem was coming from and which solution worked to fix it. Start with one solution and see what results you get and if that one doesn't work, try another one, and so on, until the problem is resolved. Now, in order for all forms of algae to grow, they require only two things; light and nutrients.

  • Lighting: 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.
    • Solutions: Only use bulbs that are designed for aquarium use, run the lights 8 to 9 hours a day, and following the basic wattage rule of thumb, try different types of bulbs to increase the intensity and 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.
    • Nutrients: Phosphates (PO4), DOCs (Dissolved Organic Compounds), and nitrates (NO3) are primary nutrient food sources for red and other slime algae.
      • Phosphates (PO 4 ) are commonly introduced into aquariums by means of using unfiltered fresh tap water, and through many aquarium products that may contain higher than normal concentrations of this element, 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.
        • Solutions: Use RO/DI filtered make-up water, a high-quality sea salt mix, and be aware of the elements contained in other common aquarium products you may be using.

    Red Slime Algae Causes & Cures

    Allowing excess DOCs to accumulate in an aquarium, in turn, gives rise to nitrate (NO3) problems. However, nitrates can also be introduced in the same manner as phosphates, and because it is the final byproduct produced in the nitrogen cycling process, it 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.

    • Solutions: Practice good aquarium maintenance care routines! This includes keeping the substrate clean, cutting back on feedings, regularly rinsing, rejuvenating or changing any type of filtering or absorbing materials (such as filter flosses, cartridges, bio wheels, sponges and carbon), performing regular partial water changes, and for DOCs in particular, adding a protein skimmer (read reviews & compare prices). For those with systems that have been running for some time and use wet/dry trickle type filters, the bio media in them, especially bio balls, are real nitrate factories, and therefore should be carefully rinsed and cleaned periodically.
    • While most hermit crab and snails won't eat this type of algae, the Left-Handed or Dwarf Zebra Hermit Crab has been known to peck away at it in an aquarium. To help keep the aquarium bottom clean and tidy, add some tank friendly algae/detritus eating hermit crabs, one or two true crabs, shrimps, or other good substrate sifting tank janitors, or a fish. Scott Michael recommends the Orange-Spotted Sleeper Goby (Valenciennia puellaris) as being the best.
    • When adding live rock, take the time to cure it properly
    • Important Note: If your tank is still cycling, DO NOT add any new animals, do ANY water changes, or perform ANY MAJOR substrate or filter cleaning tasks, other than to change dirty pre-filtering materials and/or to QUICK siphon stuff off the bottom, until the tank has COMPLETELY FINISHED cycling. Because this type of algae does not attach well, it can easily be peeled off and removed by light siphoning, with larger floating pieces being removed with a net, or turkey baster.

    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 which you can find in top Red Slime Algae removers to cure the problem quickly (within a day or 2). However, many of these types of treatments appear to only solve the symptom (the slime algae), not the underlying problem(s). Cyanobacteria are a form of bacteria, and 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. Use these types of treatments cautiously!

    By putting into action any of these solutions, as the growth sources are being eliminated you should see a gradual decrease in the growth of the slime algae. In the meantime, while you determine and correct the actual cause underlying the problem, the unsightly algae can manually be removed as mentioned above.

    One final interesting note is that because slime algae consume nitrates, often when aquarists perform nitrate tests, the readings come up as normal. Don't be deceived. If you were to remove the algae temporarily before putting into action any of the above solutions, in all likelihood, you will see a rise in the nitrate levels in the aquarium.

    It's a Catch 22. The nitrates have actually been there all along, but unreadable as the algae is feeding on it, therefore the nitrates appear to be in check. This applies to many other forms of algae as well!