No reefkeeping blog would be complete without an article on the “red slime algae”, which isn’t actually an algae at all.

Cyanobacteria is a photosynthetic blue-green bacteria (note the word “cyan” in the name) that typically appears as a reddish-purple in a saltwater aquarium.  Of course, it can also present as blue-green or brown.  I have even seen some appear almost black.  This darkening is caused by higher light exposure.  Those of you with current cyanobacteria outbreaks have probably seen this:  the aquarium appears fairly clean in the mornings…and by evening it is overrun with cyanobacteria. 

Cyanobacteria thrives in high nutrient, high light, low flow conditions.  In order to overcome cyano outbreaks, the source must be identified and reduced/eliminated.

Low Flow:
This is usually easy to determine.  Aquariums with cyano outbreaks in areas behind rocks or other stagnant areas are usually suffering from low flow.  Increasing the flow to these areas may quickly solve the problem.

High Nutrients:
If the cyano is spread rather evenly throughout the aquarium, then the problem is likely the water.  Cyano feeds off excess nutrients in the water column.  Test for nitrates and phosphates to determine if there is an obvious problem.  If these tests register low, then there may still be a phosphate problem as test kits can only register what is actually in the water column (not how much macroalgae is using out of the water column.)  Increase water changes (using RO/DI salt water), decrease fish feedings or quantity, increase skimming, and increase carbon filtering.  Make sure all mechanical filtration (filter socks, sponges, etc.) are cleaned well, and siphon out detritus in dead zones.  Use caution in siphoning deep sand beds due to the hydrogen sulfide that may be released.

Improper/Poor Lighting:
If the cyanobacteria appears mostly on the tops of your rocks (facing the light), then the cause is probably improper or poor lighting.  As bulbs age, they move toward the red-end of the spectrum which fuels cyanobacteria more.  Changing the bulbs to new ones should alleviate or reduce the cyano within a few days.

Another way to cope with cyanobacteria is to completely eliminate light on the aquarium for approximately 3 days.  The cyanobacteria dies off without light and has trouble regrowing.  Many aquarists have used this technique with great success, but I urge caution in attempting this method.  The pH can drop rather low so monitor it and leave the sump light on to help with the drop. 

Antibiotic Use:
I have met very few patient reefkeepers, so the use of antibiotics (usually erythromycin) is quite prevalent.  These antibiotics are usually found under “cure-all” type product names advertised as reef safe.  I warn of the use of these products as they are antibiotics.  They will kill the cyanobacteria as well as all the beneficial bacteria in a reef aquarium.  In essence, the aquarium may recycle or even crash.  Think back to the aquarium’s first days of cycling.  During that time, bacteria were duking it out for dominance.  In this case, cyanobacteria won.

Carbon Dosing:
Dosing vodka, sugar, and/or vinegar can cause an initial cyanobacteria bloom since the purpose of carbon dosing is to…well…feed bacteria.  As stronger but less opportunistic bacteria are fed these carbon sources, they will eventually outcompete the cyanobacteria.  However, if a cyano bloom occurs, it is best to slightly reduce dosing and eliminate other possible causes of cyano.

Speaking of epic gladiator bacterial wars, rather than killing all bacteria, why not introduce some new players to the fight?  Over time various bacteria dominate, and if a reef becomes too stagnant, the bacterial diversity diminishes.  Introducing new bacteria (through new live rock, coral, sand, or even those bacterial dosing products) can provide stiff competition for the cyanobacteria.

Diatoms vs Cyanobacteria – Choose Your Poison:
One of the most interesting articles I’ve read discussed dosing silica into a reef aquarium to encourage diatom growth.  Remember those brownish dots that covered the aquarium when it was a few days old?  Yeah, that stuff.  Silica feeds the diatoms which are able to outcompete cyanobacteria according to Dr. Randy Holmes-Farley (article link). 

Personal Experience:
Sadly enough, the photo in the introduction to this blog was our own 150g.  The first time we set it up, we used all dry rock and dry sand to avoid the typical pests.  Unfortunately, at the time, we didn’t realize the majority of bacteria needed to start an aquarium cycle successfully was found on surfaces…particularly on rock and sand.  All we did was use water from another aquarium.  The cyanobacteria had little competition, so it flourished in a brand new aquarium (new lights, new unpolluted sand/rock, and very high flow).  After months of struggling with cyanobacteria, it dawned on us that there were no other bacteria to outcompete the cyano (yes, hindsight is 20/20.)  We selected the Prodibio product line for bacteria as we wanted to keep our aquarium pest free.  I was skeptical with the first dose, and by the time rolled around for the second dose (2 weeks later) I was losing hope.  After the second dose, the cyano quickly started disappearing.  Within a few more weeks it was entirely gone.  I continued dosing, and I could always tell when the next dose was near:  cyano would crop back up on a rock or two.  Within a year, the cyano was fully gone.

While I recommend being very cautious on what is introduced to an aquarium, there is a “reasonable” point.  We blew past that point and didn’t look back.  Lesson learned.



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  1. John

    Boy has your 150 changed since that picture was taken 😛

    1. admin

      LOL!! Yes, it sure has! Thanks for the chuckle! 🙂

  2. Amanda McClellan

    I have a 75 gallon reef tank and we have been battling really high nitrates since the restart of this tank. Restarted this on Christmas of 2016 and only put 2 clowns in about a month ago, now suddenly we have this black slime covering the top of all the live rock, there is about 200 pounds of it in the tank with about a 3 inch sand bed. I moved the sand around a bit in the front and now the black slime is on the sand bed where I messed it up, and only in that area. Fish seem fine right now but I really want to get rid of this stuff and lower my nitrates. We do about 10 gallon water change weekly. We use bottled water for these changes with a TDS of about 3. I used Chemiclean last week and it looked better for about 2 days then came back even worse! Help! I just ordered the Prodibio reef stuff.

    1. admin

      Hello! I’m sorry to hear about your cyanobacteria woes! That is quite a lot of rock for a 75g (200lbs?) I’m glad to hear you’re using water with a TDS of only 3. What are your parameters? What is your ammonia, nitrite, nitrate, and phosphate? What lights do you have, and what is their cycle? What flow do you have (powerheads, return pumps, and their sizes)? Unfortunately, cyanobacteria is quite common for new tanks, but it should resolve quickly if there are no major underlying issues.

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