At times, the amount of dissolved organic material in the aquarium's water increases to such a degree that enough nutrients are present for certain types of algae to grow. In hobby literature this would be referred to as the presence of "excess" nutrients, these excess nutrients leading to the appearance of Cyanobacteria or blue-green algae (amongst others, as depending on the kind of nutrients present and their mix, other algae may appear as well).
The term "nutrients" can be confusing, as nutrients do not only include organic material that has dissolved in the water but also nitrate, phosphate and silicate based compounds that when present can shed the nutrient portion of the compound and set it free so algae can uptake it and appear on grow or continue to grow. These compounds and their break down components are called "nutrients" for obvious reasons: algae feed on them.
All of these can be present in various forms as they are not there as just the NO3 chemical representation of nitrate (the same applies to the others as well).
D.O.C., or dissolved organic carbon, is just another way to express dissolved organic material that is present in your aquarium's water. It comes from the breakdown of organic material (food, fish slime, algae, invertebrate slime etc. as seen later).
All these terms are often used interchangeably in articles and books. Be aware that when D.O.C. starts to build up in the water, whatever term is used does not matter, it always refers to the same condition: their concentration is too high, and that high concentration soon gives rise to the appearance of red slime algae (or Cyanobacteria and the other names used for this same type of undesirable algae).
For the purpose of this article we are dealing with dissolved organic material only so we will not deal with the others and will therefore not mention (or infrequently mention) green undesirable algae and diatoms or brown undesirable algae.
As indicated above, red slime algae, blue green (Bluegreen) algae and Cyanobacteria all refer to the same type of undesirable growth in your aquarium, that red patch or those red patches that are covered with a velvety type material (exuded by the algae) that is soft, can easily be removed and blows away into the water when a medium to strong current passes by them.
Many hobbyists have struggled with these types of algae and have experienced times when they were present and times when they are not.
It seems that they appear at certain periods of the year and not at others. This is kind of baffling to hobbyists, but is easily understood when one considers that water quality changes and that at certain times of the year raw water used to add to the tank is of a different make-up, and may very well contain more organic material than at other times.
This appears to be accentuated, and be more prevalent, in agricultural areas for instance, and may have to do with fertilizer by-products leaching into the water reservoirs where your raw water comes from. Not only can such type of water contain higher amounts of D.O.C., but often it contains higher amounts of PO4 as well.
This will then result in the appearance of not only red algae, but green ones in addition (these can be patchy, filamentatious, filamentous, hairy and so on).
Besides the source just mentioned, the raw water you use, D.O.C. also is the result of any of the following conditions:
What one often finds is that several of these causes happen in the tank at the same time. The resulting amount of organic compounds that dissolve in the water is therefore high. Note also that this all happens as a result of normal metabolic processes that take place in the aquarium and that you, as a hobbyist are, actually, mostly unaware that they are, in fact, taking place.
Metabolism generates by-products and these decompose and mineralize, breaking down into other compounds, some of which are organic in nature, hence the increase in D.O.C.
Although there is no easy way to measure D.O.C., at least not for the hobbyist, one characteristic of the water quality that is a good indicator of increased D.O.C. is a reduction in the dissolved oxygen content of the water (indeed metabolism and catabolism use up oxygen).
How is D.O.C. removed from the tank?
Protein skimmers are excellent devices to do so. Unfortunately the following situations can occur:
All the above are examples of a certain amount of organic material being removed, but the amount removed is smaller than the amount produced. The net result is that the amount of D.O.C. rises. Let it rise for long enough and undesirable algae will appear, guaranteed.
Testing done by adding organic material to a tank on a daily basis soon resulted in the appearance of red algae (Thiel, 1989).
Organic material is made up of protein. Protein breaks down into many organic compounds, some of which are obvious and some of which are not. Amino acids make up protein and are an example of the former. Complex organic compounds that break down when protein disintegrates are examples of the lesser obvious ones.
What has happened here is that over a period of time, due to problems related with the skimmer or any of the other reasons mentioned, the amount of such organic compound has slowly, but surely, increased.
This increase is the nutrient base for the Cyanobacteria or red algae and is the cause that they suddenly start appearing in the tank, especially when strong lighting promotes their growth, and when temperatures are propitious to their proliferation (higher temperatures make them grow faster).
This may also explain why it is not uncommon to see more of them in the summer than in the winter. Indeed, during summer times your tank's temperature may rise a few degrees. Those few degrees are actually enough, in combination with the presence of the D.O.C. to result in outbreaks of red slime algae.
What should you do?
Obviously, it would seem clear that what needs to be done is to reduce the amount of dissolved organic material dissolved in the water. This is easier said than done though. You have a skimmer and just because you decided to do something does not make the skimmer work that much more efficiently that all excess D.O.C. is removed in a matter of days and that red algae disappear. Gee, we all wished it was indeed so but, unfortunately, it is not.
Moreover, when red algae die off, they make even more nutrients while they decompose and break down. These new nutrients then form the base for more red algae to grow.
That is the reason that as many as possible of the Cyanobacteria should be siphoned out, so they do not die off in the tank and pollute the water even more. Water changes will help reduce the amount of D.O.C. but sometimes this and siphoning algae out is not enough to get the red algae to disappear completely.
This is the reason that an additional approach can be taken: oxidize the organic material using a strong oxidizing compound to remove the organics, and deplete the nutrient pool that the red algae need to grow. Using, for example, compounds such as permanganates can do this.
Thiel Aqua Tech makes such a product called Redox + Liquid. Used as directed it will burn out, so-to-speak, the organic material and lower the nutrient pool for the red slime algae. Using this product in combination with siphoning out the dying ones, will ensure that your red algae problem gets solved rapidly.
Of course, to prevent it from reoccurring you will need to take measures that will prevent the D.O.C. from building up again. This may entail using a more efficient skimmer, reducing feeding, increasing the amount of water changes you perform, and so on. The key is that you do something or sooner or later the red algae will reappear.
One method used by hobbyists consists of first eradicating the red algae with Redox + Liquid, using the recommended treatment dosages. Then they follow this treatment up with the addition of the Liquid twice every week, at the rate of 1 tsp. per 50 gallons, once in the morning and once in the evening (meaning two days of two treatments or a total of 4 tsp. a week).
This method seems to help a lot of hobbyists control the reappearance of Cyanobacteria and is therefore recommended to those who think they need extra treatment to ensure that the slime algae do not come back.
Should you have any questions regarding red slime algae or regarding Redox + Liquid, please Email Me and I will be glad to respond.