Dealing with Cyanobacteria and Red Slime Algae

Copyright 1997, Albert J. Thiel

The Saltwater Library of our web site contains several articles on the basics and introduction to Cyanobacteria, or red slime algae as most hobbyists refer to them. Note from the start that even though we refer to them as red slime algae, they can appear in your aquarium in a variety of colors. The most prevalent ones, though, are the red slimy types hence the name most hobbyists use to refer to them. Then again, not all types are slimy (as the articles in the SW Library of our web site explain). It should also be noted that the slime you "see" is not the actual algae or cyanobacterium agglomeration, but what it/they exsude. The actual algae are underneath.

Once Cyanobacteria are present in the aquarium and you wish to eradicate them, or prevent them from occuring again altogether, requires a dedicated and persistent structured approach, "and" good maintenance of a particular kind afterwards (as described in this article).

The combination of such practices ensure that Cyanobacteria do not reappear after a brief period of time or even after a longer time frame because you are doing what is necessary to keep the nutrient levels they require to grow, low if not very low.

The purpose of this article is to go through the various steps and interventions you need to undertake in each of those cases. I have listed them in that order, as hobbyists usually do not intervene until red slime algae suddenly appear, often to the total surprise of the hobbyist who thought that he or she was doing everything necessary to ensure excellent water quality conditions in the tank.

A lot of preventative steps can, however, be taken to make sure that they do not show up in the first place. That is the ideal situation and should be the goal of every reef hobbyist. Although, in nature (open system), cyanobacteria play important roles in the life chain, it has not been demonstrated that they serve any necessary function in the aquarium (closed system).

Using "other" methods than the one described here "may" work, but the problem usually is that you will only have dealt with the effect and not the cause. If you do not deal with the latter, your cyanobacteria will invariably re-appear because the "nutrients" they thrive on (dissolved organic material mostly) have not been eliminated from the tank. To truly rid the tank of them, you will need to first eliminate those nutrients and then follow a "maintenance" approach, to prevent the D.O.C., or dissolved organic carbon, or dissolved organic material, from building up again and giving rise to more red algae (or other varieties of cyanobacteria).

In this case "other" methods refers to the use of different compounds to rid the tank of cyanos, or the use of antibiotics. The former usually is only a patch method and the latter may create havoc with your biological filter, leading to the appearance of ammonia, nitrite and resulting in lots of stress for all animals. Possibly, when this occurs, you may find that your tank is recycling completely because the antibiotics have destroyed your biological filtration. Recycling is a real high stressor on all animals and will usually lead to the fish being attacked and infested with parasites, creating another problem that is even harder to deal with in reef tanks. How to do so will be dealt with in another article. In essence, we will be using Vitamin C of a special kind to do so.

Eradicating cyanos, from what I described so far, may appear a little more complicated than you had imagined. It really is not. Good maintenance and husbandry keeps the water chemistry where it should be, and benefits all the tank's inhabitants. Besides, it also prevents the growth of other forms of undesirable algae (for example green slimy and filamentous ones and brown ones, generally of the diatom variety), It also prevents the appearance of other cyanobacteria, not only red slimy ones dealt with in this article.

When something goes wrong in the aquarium, the cause is bad water chemistry in just about 99 percent of all cases. The processes described here, since they improve the water quality, will result in an overall better looking aquarium, not just the disappearance of red slime algae.

To understand more about Cyanobacteria you may wish to read the free articles that are part of our Saltwater Library. Although they do not explain how to rid the aquarium of Cyanobacteria, they give a lot of introductory material that helps you understand what these bacteria/algae "feed" on, and in what forms they can, and will, appear if you let your water quality deteriorate. Presently there are three such articles in the SW Library on our web site at

It is assumed, and should be understood, that every hobbyist is aware of and knows that on a saltwater aquarium you need a good strong foam fractionator (protein skimmer). There is no way around it, you do need one. It is good practice to use one that is rated for "twice" your tank's size. It is also good practice to use a "venturi" type, or some of the newer downdraft ones that have recently appeared on the market. Running a skimmer is, at this stage, obligatory for more hobbyists. When I say most I mean the vast majority.

Whether the downdraft protein skimmers are actually more efficient in the long run than venturi types has not been demonstrated conclusively yet. The new downdraft ones certainly do an excellent job in the beginning. In my experience though, their performance may slack off, whereas venturi types do not appear to have this problem. In any event, what ever type you decide on, it must be powerful and reliable as it greatly affects the quality of the water in the tank in many ways.

Running a saltwater or reef aquarium without a skimmer is, at this point not recommended, although you will find references to hobbyists having done or doing it. This is experimental and may be even anecdotal and is not something you want to test on your tank, certainly not on your main one if you have more than one.

Protein skimmers are often greatly misunderstood. Their function and the resulting improvements in water quality are often underestimated. Foam fractionators remove many more undesirable elements from the water than the ones discussed here. As such they greatly improve the water quality. Note that besides removing unwanted chemicals, compounds and elements, they also remove good ones. That is unavoidable. The good ones can, however, easily be replaced by using complete reef tank additives such as Vital Gold and Combisan, to name only two. You can search the database of our message list on our web page for feedback from hobbyists who have used these products. They report fast and really positive experience. All you need to do is go to our web site on the main index page and, towards the bottom left, there is a clickable link to the SQL search engine you can use to search over 65 meg (at the time of this writing) of information from hobbyists.

Where do the Cyanos come from

Cyanobacteria are complex and not all "that" well understood to be honest. A lot has been written about them, but in terms of aquarium control of such algae there is little conclusive material.

In the greater majority of cases the nutrients these algae thrive on is dissolved organic material. Where does this material come from? It comes from the decay and breakdown of anything that is alive or once was alive in the aquarium.

Sources include (but are not limited to):

  • Fish slime
  • Invertebrate slime
  • Other life forms in the tank
  • Algae and bacteria
    • dead ones or
    • live ones
  • Excrements
  • Excrements that contain partially digested food
  • Uneaten food
  • metabolic and catabolic processes
  • Material on rock
    • Live rock and the life forms on it
      • alive
      • dying
    • non live rock on which and in which some life forms exist that you may or may not see because the size may be real small (again these life forms may be alive or may be dying)
  • Additives you use that are high in organic material
  • Live foods
  • and so on, indeed, this is only a partial list, but as you can see the sources are indeed numerous already.
When any of these start to decompose, break down is a better word really, organic compounds are released and mix with the water. These compounds then become the "nutrient source" that leads to the sudden appearance of spots of cyanobacteria. At first they are real small and may only appear in one or two areas. These spots quickly become larger and larger though if nothing is done to prevent this from happening. Suddenly they are visible in more than one area of the tank and are now large and distinctly unsightly. What is happening is that Cyanos are overtaking the aquarium because the nutrient levels they require to grow are now high.

You may have experienced that your best efforts to siphon them out do not seem to make much of a difference, if any. Water changes do not seem to make much of a change either. Whatever you do, the algae appear to continue to grow and reappear, sometimes in more locations, sometimes in others, often in the same ones. This process actually occurs rapidly. Indeed, you may have siphoned them out at night, only to see them again in the morning. This can, I understand, be a very frustrating experience. This is very undertandable. When one tries all sorts of remedies and none seem to yield any positive effects, the result is discouraging at best. Note though that red slime can be gotten rid of. To explain how to do so is the purpose of this article.

This article is intended for the Download for a fee area and for the NetClub area. I thought though that an introduction in the general library would give you a good idea of what you are dealing with and call you attention to the articles in the SW Library called Introduction to Cyanobacteria I, II and III. Please read them. They will give you a great deal of insight into this algae type which is considered somewhere between a bacterium and an algae as it demonstrates characteristics of both.

Let us take another look at what happens in the aquarium :

  • Any of the conditions that adds organic compounds to the water exists in your situation and probably more than one is at work, not just a single one. When water chemistry gets out of hand, the causes are more often than not, multiple.
  • The skimmer removes "some" of this dissolved organic material but more is actually produced than the skimmer is removing. This is a very common occurance. Either the load is high or the skimmer too small, or both. In either case, DOC will start to build up.
  • Given enough time, the amount of dissolved organic material reaches a stage and a concentration where the amount present starts giving rise to the appearance of red slime algae because there are now enough nutrients present to allow this growth to occur. In this respect you may wish to consult "Organic Chemicals in Waters" James W. Moore and S. Ramamoorthy, Springer Verlag, 1984, ISBN 0-387-96034-1 (cost $50.00).
  • When the increase continues more and more cyanos will grow since what is being removed is less than what is added to the tank water.
  • This accounts for the growth that seems to rapidly increase once it takes off even though you may be siphoning algae out and changing water.
  • It should be noted that the slime produced by these cyanobacteria is just about pure organic material and decomposes, adding more organic matter to the tank.
  • Of course the amount that decomposes is replaced by a new sheath of slime, and the process just continuous on and on. One dies off and decomposes, adds organics to the water, and immediately a new one develops underneath (and at the same time) that soon dies and adds more organics, a new one grows, etc. You get the picture. DOC keeps rising and rising.
  • Organics are not the only matter involved in the growth of blue-green algae (Cyanobacteria or red slime or whatever else you want to call it actually). Iron is another nutrient, but to a lesser degree than DOC or dissolved organic carbon (organic material dissolved in the water). I mentioned an interesting book earlier. Another easier one that you may wish to read or perhaps refer to if you can find it at a library is: "Inorganic, Organic and Biological Chemistry" by Philip S. Chen, Second Ed., The Barnes and Noble Outline Series, 1979. ISBN 0-06-460182-x (originally published by Harper and Row).
  • Those of you who have dealt with red and other slime algae have gone through this and know exactly what I am talking about.
  • Once the growth of red slime algae starts, rapid intervention is necessary or the whole tank will soon be covered with them and the growths will choke what is underneath, often killing life forms. Life forms that die off then lead to more decomposition and more organic matter being added to the water.
In essence, what I have found is that when the amount of dissolved organic matter builds up to a high level, sooner or later (usually sooner than later) red slime or blue-green algae will start to grow.

The key then, is to keep this amount low. This is achieved through skimming but may need to be supplemented by other means, especially if you have red slime algae present in the tank already.

Below are the suggested procedures for eradication and prevention

1. You have red slime Algae in the tank

If red slime algae are present already you will need to lower the dissolved organic level in the tank's water. This can be achieved by water changes of course, by adjusting your skimmer and making sure it removes as much of the DOC that is present as possible, or you can do so by using a potassium permanganate solution. Thiel Aqua Tech sells such a solution under the name Redox + Liquid. We used to have a powdered version but are no longer able to sell it because it is too potent. You can read up more on this product in our web site's SW Library under Product Related Articles.

Of all the methods listed above, the one what will give you the best and fastest results is the use of the Redox +. It is a strong oxidizer and will break organic material down, allowing the skimmer to remove it in an easier fashion (when using it you will see that the color of what the skimmer takes out changes).

NOTE: when you use the product, remove the following types of compounds from your aquarium's filtration system (this applies only in this section. When you do maintenance and avoidance treatment regimens of Redox + you do not need to remove these and this is explained further towards the end of this document).

  • Nitrate removers
  • Phosphate removers
  • Silicate removers
  • Activated Carbon
  • Resins
  • Products such as SuperChem, ChemiPure, ChemZorb and the like
  • Poly Filters
  • Any similar type products not covered above
  • but, most importantly, Leave your skimmer running

Performing a water change before you begin does not hurt but is not necessary and, if you do so, you must make sure that all the parameters of the water you add correspond to the parameters of the water already in the tank. If not you will be creating stress for the animals. Remember: stress must be avoided at all costs. It leads to weakened animals and outbreaks of disease. Often it leads to parasitic infestations as well. Stress is the last thing you want to introduce into the aquarium.

Follow the directions on the Redox + label and do not overdose. There is no benefit to it, only danger and stress. Perform additions several times a day at regular intervals and watch that skimmer cup. It will need servicing more than usual. Watch the skimmer too as you may need to make some adjustments to its running.

Siphoning algae out while treating with Redox + is a good idea too as that will speed the process up. Mind you, you will not rid the tank of slime algae in a day or two. You may need to treat for 5 or more days, several times a day, depending on how much dissolved organic material is in the tank.

If you are not at home and if no one can do it for you, the suggested method is for you to add one dose early in the morning, and a dose when you get home and then another dose 2 or so hours later. That gives you 3 a day. If you can squeeze two in in the morning that is even better. Make sure though that they are added two hours apart from each other. Maybe if you get home in the late afternoon and go to bed late you can add 3 in the evening. Say 1 when you get home and then one every 2 hours after that.

It is not uncommon, after a day, or even several days of treatment, to still see red slime algae grow. What that means is that the amount of DOC is real high and that even though you treated the tank to lower DOC (dissolved organic carbon or dissolved organic material or in general dissolved organics), algae are still finding enough nutrients to grow.

The obvious solution is to continue treating with the permanganate solution until the DOC gets so low that the red algae do not reappear. Continue to siphon out whatever grows back at the same time. This, as indicated earlier, speeds the process up.

As you continue this treatment, as per intructions on the bottle of diluted permananganate, the DOC will get lower and lower and the slime algae will start to diminish at first and then disappear altogether.

After they have all gone, continue the treatment. However, instead of adding several dosages a day a two to three hour intervals, add a dose in the morning and one in the evening. Do so for another 4 to 5 days and your water should be low enough in DOC to prevent slime algae, or blue-greens or cyanos, or whatever you call it, from regrowing or reappearing.

NOTE: as you continue the treatment and as less red algae appear, you can diminish the amount you add to the tank. Indeed the same amount of permanganate solution will have a greater impact on the DOC as there is less of it in the tank's water.

It is not really possible to give you an exact schedule because the amount of DOC dissolved in the water is what determines how long you need to treat for and at what dosages (note ... never go higher than the dosage recommended on the label).

Additions of Redox + are at two to three hour intervals. At first, 2 hours is fine and recommended. After two to three days you may wish to go to adding it every 3 hours. The frequency really depends on whether or not you still have lots of slime algae in the tank, or not. If the number has diminished considerably, go to adding the permanganate solution every 3 hours. Then, as the amount diminishes even more, go to the morning/evening addition.

I wish I could give you an exact schedule to follow, but that is not possible unfortunately. As I stated the amount of dissolved organic material determines the amount and frequency of the additions of Redox + Liquid. In this article I mention that product. There may be others out there that can do the same thing but, at the time of this writing, at the end of April 97, there were none.

Potassium permanganate, even in diluted form is a real strong oxidizer. You need to adhere to the dosages recommended or the product will create an environment that is too high in pH. It is best, therefore, to play it safe. Although the dosages recommended are, aquariums differ in how they react to the addition of Redox + liquid. What would be a good idea is to to the following:

  • Use half of the recommended dosage
  • Two hours later add that same dose again
  • Observe the animals
  • If they close up somewhat or completely you do not want to raise the dosage.
  • If they remain open you can use more (say 75 % of recommended dosage) and observe again.
  • You can raise to the recommended dose is you see no adverse effects at 75 % of the recommended dosage.
  • The amount you add at one time is less important than the frequency and the continuation of the treatment. That is why adding less but staying with a two hour interval for each addition will eventually yield the same results: oxidation of organics and their removal from the tank.

The key in this section of treatment and eradication is to continue for a few days with adding the solution, even after all slime algae have disappeared to ensure that the DOC is really really low and that no algae will regrow.

There is another important note: if the DOC built itself up to begin with, you should try to determine why. Reasons include:

  • Overfeeding
  • Skimmer not functionning properly or efficiently
    • Venturi valve needs cleaning
    • Wooden airstones need changing
    • Air pimp is not delivering enough air
    • skimmer is too small for the load in the tank
  • Bio load is too high
  • Additives used are adding too much organic material
  • and so on. These are only a few.
We are now basically at the stage where you have eliminated what ever cyanobacteria were in the tank and you have lowered the DOC to acceptable levels. The next step is to keep the DOC low. This is achieved by ensuring optimal skimming and not overfeeding and not overloading the tank.

Note though that in many instances you cannot simply start to make changes to the tank that easily. This being the case, we enter phase II of the maintenance.

2. Maintenance Phase

What is done in this phase is really very simple: add Redox + Liquid to the aquarium in the AM and in the PM, once each time and do so several time a week.

How often you should do so depends on the load in the tank. If the tank carries a real heavy load, add 1/2 the recommended dose every day, once in the AM and once in the PM. In cases of medium loads, do so every other day. In low load cases once a week will be sufficient.

Should you do this every day, forever? The anwser is no. Do so for 2 weeks in a row and then skip a few weeks (say 8-10) and then do another treatment of one week. Time will tell how often you need to perform the treatment. You will get a feel for it after using the product and gauging the condition of your aquarium.

The method I use for maintenance is: half the recommended dose twice a week. On each occasion I add that quantity in the AM and once again in the PM. I do so every week. I have found that this twice a week double treatment keeps DOC low at all times and that I never have problems with cyanobacteria.

Whereas a number of products are removed when you do the continuous, several times a day treatments, you do not need to remove these products when you are in the maintenance regimen. You can leave them in.

What I recommend is that if you keep your compounds in pouches or micron bags that they be rinsed off once a week to remove slime and detritus as this ensures that water can pass through them properly. Mind you, this procedure is recommended whether you use Redox + or not. It is a good idea to do so as it maximizes contact between the water and the compounds and allows them to work more efficiently for you.

I realize that reading through all this you may get a little overwhelmend by all the information. If you have any questions what so ever, feel free to Email Me directly from this document. I will be glad to answer your questions within a reasonable amount of time as I am not online 24/7.

The maintenance phase is really not complicated. All you need to do is find the regimen of additions of the liquid that fits your aquarium best and then adhere to it, always making sure that your skimmer is running at optimal ranges. If it does not, you will find that you will need more liquid to achieve the same result because the skimmer is not operating as well as it should. Clean the venturi regulary, change airstones as needed etc... and ensure that your protein skimmer IS running as it should.

3. Avoiding the Appearance of Cyanobacteria

Avoiding the appearance altogether is similar to the maintenance procedure but what is not needed is the initial more frequent additions.

Go to the section where you add Redox + either twice or more times a week, twice a day, depending on the load and the skimming efficiency you have on your tank.

How often can be determined by measuring the DO in the tank (dissolved oxygen level). Indeed, when the organic load is high, the DO will tend to fall because of the organic decay that goes on in the tank. What you can do is measure the dissolved oxygen level. Write the result down. Now start treating with Redox + for a few days, using the AM and PM addition method. Then, after a few days measure the DO again. It should have risen. When you continue and the DO fails to rise you know that you have achieved maiximum DOC removal. When the DO starts to fall again, a few treatments will bring it back up.

Based on the treatment frequency (DO falls and you treat intervals) you kind of know, without testing when you should perform a few treatments.

Note that doing so will keep your tank free of Cyanobacteria year round but you need to find the right treatment regimen. Doing so by using the Dissolved oxygen method is the easiest method I have found. It really allows you to determine when treatment is necessary.

The avoidance of Cyanos regimen is easy to follow, and doing so will save you from having to deal with sudden outbreaks that require more radical intervention.

Whereas the bottles indicate a recommended level, it is always a good idea to "test" as described earlier, to ensure that you are not adding too much at one time. Frequency is indeed more important than quantity (as explained).

If anything is unclear, or if you need more info, Email Me Please.

Update April 19, 1997 for DL/Fee and NetClub™ areas of the Thiel Technologies web site

Copyright Albert J. Thiel, April 1997