Green Dust Algae, also known as GDA, is a type of single-celled algae that commonly appears as a green powdery coating on aquarium surfaces.
GDA on the aquarium wall is definitely an unpleasant sight, but it’s actually quite harmless and in many cases, can be easily removed.
In this article, I have gathered everything known about Green Dust Algae, including the various methods that aquarium enthusiasts have employed to combat it. These methods range from blackouts and chemical treatments to simply allowing the algae to run its course without intervention.
What are Green Dust Algae?
To be completely honest, nobody really knows for sure what kind of algae it is. The term “Green Dust Algae” is not taken from scientific terminology but rather from aquarium enthusiasts’ jargon.
There are now two primary theories: Green Dust Algae may belong to the Chlorococcum or Ankistrodesmus genera.
Both belong to the group of green single-celled algae that can exist in freshwater environments including lakes, ponds, and rivers. They can also be found in soil, on rocks, and as free-floating organisms.
Unfortunately, there are numerous species of green algae, making it basically impossible for ordinary aquarium hobbyist to identify which precise type of algae is growing in their aquarium.
These algae are photosynthetic and use chlorophyll for energy production. Like other green algae, they contain chloroplasts that give them a green color.
How to Identify Green Dust Algae
As the name suggests, this type of algae appears on the glass as a green deposit, which can quickly accumulate and become so dense within a few days that you can’t see through the glass anymore.
Generally, Green Dust Algae settles evenly and forms a carpet-like layer.
Reproduction of Green Dust Algae
Depending on the genera, Green Dust Algae have 2 ways of reproduction:
- Asexual reproduction takes place by the formation of zoospores. The cell contents divide and separate into 8-32 zoospores that swim in the water column. They attach themselves to smooth surfaces, such as glass, and the cycle repeats itself.
- Sexual reproduction by isogamy in which both the gametes involved in fertilization have similar morphology.
Note: Genus Ankistrodesmus reproduces only asexually, whereas genus Chlorococcum can do both types.
It usually takes about 3 weeks to complete the full development cycle of Green Dust Algae. At the beginning of this process, spores transform into cells that settle on glass or other surfaces. After that, these cells start to detach on their own, forming a green film.
Difference Between Green Spot Algae and Green Dust Algae
| Green Dust Algae | Green Spot Algae | |
| Morphological | Green dust-like film | Forms tiny, circular green spots |
| Texture | Smooth | Rough |
| Adhesion | Easily wiped or scrubbed off | Very tough to scrape off |
| Growth rate | Extremely fast | Medium-fast |
Distinguishing between the two can be done by touch: Green Spot Algae feels rough or hard when touched, whereas Green Dust Algae feels smoother and can be easily lifted off.
Additionally, green spot algae are more difficult to scrape off surfaces, while green dust algae can be wiped or scrubbed off with relative ease. However, if left untreated for a prolonged period, green dust algae can also form a thick layer that becomes harder to remove.
Furthermore, in terms of growth rate, Green Spot Algae generally exhibit slower growth compared to Green Dust Algae.
Causes of Green Dust Algae
The reasons for the presence of Green Dust Algae in our tanks are the subject of various rumors and arguments on forums. Nonetheless, this question remains poorly understood. It is known that such algae can suddenly appear in both newly set up and established aquariums, as well as high-tech tanks and low-tech tanks.
Based on my observations and the experiences shared by other aquarists, it has been noticed that the occurrence of these algae in different situations can be influenced by the following factors:
- Sudden increase in temperature
- Long photoperiod (>10 hours)
- Increase the concentration of salts in the water (for example, after treating fish)
- Slow plants growth (no fast-growing plants), meaning that there is an excess of nutrients but a lack of plants to consume them
- Phosphorus and nitrogen disbalance (for example, a high concentration of phosphorus (>0.8 g/L) coupled with a low concentration of nitrogen (<5 mg/L)
In most cases, the growth of Green Dust Algae is stimulated by a combination of factors rather than a single factor.
| It should be noted that Green Dust Algae is always present in the aquarium as part of its microflora!
In most cases, it appears as a light haze on the front glass, this is considered normal. However, if certain factors coincide, the reproductive process enters an active phase, and within a few days, a dense powdery deposit forms on the aquarium walls. |
Are Green Dust Algae Harmful to Fish or Shrimp?
Green Dust Algae are not harmful to fish or shrimp. On the contrary, shrimp and some fish species will gladly eat these algae.
Are Green Dust Algae Harmful to Plants?
No, Green Dust Algae generally do not harm plants directly. Unlike some other types of algae, these algae do not smother the leaves of aquatic plants. In most cases, plants and even decorations remain clean.
Nonetheless, if the growth of Green Dust Algae becomes too excessive, it may cover the leaves, hindering the plants from receiving the necessary light for photosynthesis. As a result, the affected plants will suffer from nutrient deficiencies, become susceptible to diseases, and ultimately decay.
Do not panic, this does not happen very often.
How to Remove Green Dust Algae
On the internet, aquarists from all around the world share methods for battling these algae and discuss the ones that did not work for them.
I would like to stress once again that Green Dust Algae is a general term that could refer to dozens if not more, different kinds of green algae.
Therefore, it is impossible to identify a single approach that would work for all types. Because of this, what worked for some people might not work for others. Anyway, it is better to know all your options, right?
1. Manual removal
Of course, it can be difficult to look at an aquarium that is completely green. As a result, many aquarium hobbyists turn to daily cleanings to combat these algae.
| It should be noted that this method will not provide long-term results if you haven’t identified the underlying cause of their appearance. |
Nevertheless, when manually cleaning them, there are a few tips to keep in mind:
- drain maximum water from the aquarium.
- use cotton pads instead of plastic cards or even your fingers. While Green Dust Algae are easy to clean, using pads helps minimize the spread of spores in the aquarium.
- clean the biofilm from the glass. It is a thin transparent layer. Green Dust Algae do not settle on clean glass; they settle on this layer. Use Hydrogen peroxide (link to check the price on Amazon).
- do a water change.
It will give you a few days of rest.
Efficiency: Low
2. Be patient
It is difficult to pinpoint the originator of this method. According to some sources, it was renowned aquascaper Takashi Amano, while others credit biologist and aquarist Tom Barr.
The main idea behind the technique is to do nothing but wait for 3 – 4 weeks. Do not even do regular water changes.
What’s the purpose and why does it take so long?
The answer is simple: these algae must go through their entire development cycle, which takes around 3 weeks, as was previously explained.
After that, they will start to turn a little yellowish. GDA do not peel off, but rather thins out and start gradually disappearing in patches.
The algae on the glass surfaces can then be manually removed during a significant water change.
- drain maximum water from the aquarium,
- wipe the glass with hydrogen peroxide,
- perform a thorough water change,
- then darken the aquarium for 24 hours,
- wipe the glass again a few times afterward for the next few days,
- do another water change, every 2 – 3 days.
Now, why shouldn’t we clean during this period?
The reason is that when we clean the glass to remove Green Dust Algae, they tend to disperse throughout the entire aquarium, essentially initiating a new cycle of their reproduction.
Efficiency: High
3. Phosphorus and Nitrogen Balance # 1
It may seem a little strange, but this method has actually proven to be quite effective against GDA.
- You need a macro ratio of 1:14-16. Macros should be added based on nitrate 7.
- Micros containing iron should not exceed 0.07.
- If possible, reduce the amount of blue spectrum in the lighting.
Efficiency: High
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4. Phosphorus and Nitrogen Balance # 2
- Do a 50% water change.
- Add nitrogen fertilizer at the old dosage.
- Clean the glass every day to remove GDA.
Note: Going forward, the presence of these algae acts as an indicator, making this step crucial in the process. - Every day, add PO4 (phosphate) at a rate of 0.5 mg/L until the green algae no longer appear.
- After the green algae disappear or after 7 days, do a 50% water change.
- If the green algae persist, double the PO4 dosage to 1 mg/L after the water change and continue until it disappears.
Calculate the total amount of phosphorus that was added at that time after the Green Dust Algae have disappeared and you have performed a water change. This amount should be divided by 7 days and distributed equally throughout the week.
If you reach the 3rd week and GDA are still present, it may be wise to stop the process as there may have been a mistake somewhere.
Efficiency: Medium
5. Darkening the Tank
Green Dust Algae are photosynthetic. They thrive in well-lit conditions. Thus, adjusting the lighting duration and intensity can help discourage its growth. Consider reducing the lighting period or investing in a timer to regulate the light cycle.
Potential problems:
- It can harm plants that require a lot of light but they should survive.
How to do a Blackout:
- If you have fish, feed them a regular amount of food. Do not overfeed. If it is a shrimp tank, you may even skip this step.
- Turn off the CO2.
- Do not add fertilizers during the blackout.
- Increase the oxygen supply. Install a new air-pump if needed.
- Cover the tank with a thick blanket. It must be completely dark in the tank.
- Leave the tank this way for at least 3 days.
- Open up half of the tank and leave it like this for 30 minutes. Let your fish adapt.
- Remove the blanket completely and wait another 30 minutes before switching the aquarium light on.
- Do a water change.
- Manually clean GDA.
- Change your light timer to 4 – 5 hours for the next week.
Efficiency: Low-Medium
6. Lower Photoperiod
In some cases, good results of removing Green Dust Algae can be achieved by simply reducing the photoperiod.
- Shorten your photoperiod by 1 hour.
- Wait for 3 – 5 days.
- If you do not see results – decrease the photoperiod by an extra hour again.
The algae may die off on its own and disappear.
Efficiency: Low
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7. Erythromycin
- Do a large water change (50%) and manually remove as much GDA as possible.
- Add 500 mg of erythromycin per 100 liters of aquarium water.
Note: It is important to add it specifically at night when the lights are turned off. Do not add it during the day. - Continue with your regular routine during the day, such as fertilizing the plants as usual.
- Repeat this process for 4 consecutive days.
- After the 4th day, do another water change.
Note: The situation with erythromycin is a little tricky because some people have used it against GDA successfully while others haven’t had any luck. One theory suggests that in cases where it was effective, it might have actually been targeting cyanobacteria instead. Сyanobacteria can also be easily identified by its slippery, slimy texture, carpet-like coverage on surfaces, and strong, unpleasant odor (GDA do not smell).
Efficiency: Low-Medium
8. Green Dust Algae Eaters
Some species of fish and invertebrates will help in controlling the presence of GDA in your tank:
- Freshwater snails
Although most freshwater snails will eat GDA, the most effective include Theodoxus snails, Hairy snails, Marbled Limpet Nerite snail, Horned Nerite, and Nerite snails.
- Fish
Chinese Algae Eater, Siamese Algae Eater, Otocinclus Catfish , Borneo sucker, Butterfly Hillstream Loach, and certain plecos (Hypostomus spp.) are also known to consume GDA.
- Dwarf shrimp
Although Dwarf shrimp can eat GDA, they will have difficulty cleaning these algae off glass surfaces due to their limited ability to grip onto them.
| It is important to note that the effectiveness of algae-eating animals may vary. Additionally, they should be chosen and introduced to the tank with caution to ensure compatibility with other tank inhabitants. |
Efficiency: High
Are UV Sterilizers Good Against Green Dust Algae?
Not exactly, UV sterilizers are primarily effective against free-floating bacteria, algae, and microorganisms that come into contact with the bulb.
Green Dust Algae can reproduce on surfaces and in the water column making UV sterilizers less effective against them. However, combining with other methods, they can be beneficial.
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Hydrogen Peroxide Injections vs GDA
Injecting hydrogen peroxide will not have any effect on these algae. In my opinion, it is not even worth trying.
Hydrogen peroxide can be useful and effective only when manually removing Green Dust Algae and directly treating the biofilm on which they grow.
Water Flow vs GDA
Some aquarium enthusiasts have reported that altering water flow helped in their cases, as these particular algae are typically absent in fast-flowing water.
However, based on my own experience, I can say that it did not help me at all.
CO2 vs GDA
If you have a planted tank with sufficient lighting, the use of CO2 is essential for the growth of your plants.
Reducing CO2 levels would be counterproductive as it would slow down plant growth, which is what we need to combat any type of algae, including this particular one.
That’s why I believe that if you already have CO2 injection, there is no need to change anything in this aspect.
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In Conclusion
Don’t be surprised and, moreover, don’t panic if Green Dust Algae suddenly appears in your aquarium. In reality, these algae are perhaps the most harmless, and, aside from their unattractive appearance in the aquarium, it poses no harm.
In general, Green Dust Algae is still poorly understood, and even among aquarium enthusiasts, there isn’t a consensus. Some hobbyists might have their own theories, but there is no unified opinion.
Although in most cases it can resolve itself after a couple of water changes, one of the most popular and effective methods to combat it is to leave it alone and let it go through its development cycle, after which it will disappear. Another excellent method is to introduce phosphates and nitrates in a ratio of 1:15.
Of course, there are other methods of dealing with it, but their effectiveness is somewhat lower.
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- Types of Algae. Best Algae Eating Team
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