How to Remove Green Spot Algae in Aquariums

How to Remove Green Spot Algae in Aquariums

The presence of nuisance algae can be quite nasty in fish tanks due to the ability of these organisms to form ugly patches all over your tank, hence making it unsightly and in some cases— unpleasant for aquatic life.

There are several types of nuisance algae that plague freshwater aquariums, but this article concentrates on the Green spot algae (GSA) which tends to leave tiny green spots on the aquarium glass, ornaments, substrate, and foliage of mostly slow-growing plants.

To begin with, it is normal to see little traces of these algae in your aquarium. However, before it starts to spread vastly on your plants’ leaves, tank walls, and décor, you need to take necessary actions to keep in check and/or halt its growth entirely.

Keep reading to find out more about the Green spot algae, its causes, and effective ways to eradicate them from your home aquarium.

What is Green Spot Algae?

The Green spot algae is a type of green algae from the Coleochaete; which is a genus of parenchymatous charophyte green algae in the order Coleochaetales.

These green algae are capable of covering diverse surfaces in the tank such as the tank glass, décor, plant shoots, and even the filter if left untreated.

Green spot algae’s growth can be quite tricky. It easily forms on the leaves of a plant and gradually spreads to the rest of the body and to other plants in close proximity as well.

Green Spot Algae - GSA (Choleochaete)2

How to Identify Green Spot Algae

As its name suggests, the Green spot algae can be easily identified from its small, circular, bright green spots. These numerous green spots are nearly flat and hard; this can be felt when you run your fingertips on its surface.

Green spot algae form these small green spots on hard surfaces such as live rocks, driftwood or bogwood, and the aquarium glass. In a similar fashion, Green spot algae will grow on the leaves of slow-growing plants in your tanks, take for instance Anubias, Bucephalandra, etc.

The spots formed by Green spot algae are clearly light green in color, especially the ones attached to the walls of your aquarium. Whereas those that grow on the leaves of slow-growing plants may appear as dark green spots. 

This is a tiny alga (1 mm or 0,04 inches in diameter) but when left go thrive, the green spots will merge to form a blotchy carpet on hardscapes, at this point, it wouldn’t look like mere spots anymore.

These algae have a slow growth rate, unlike other algae species.

Reproduction of Green Spot Algae

Biologists concluded that Green spot algae have 2 ways of reproduction:

  • Asexual reproduction takes place by the formation of zoospores
  • Sexual reproduction by oogamy. After fertilization, a sterile jacket is formed around the zygote for its protection.

Green Spot Algae vs Green Dust Algae

Both Green spot algae and Green dust algae are known to be unwanted/unfriendly to freshwater tanks because of how appalling they look and their ability to form green coats on various surfaces in the tank.

They grow and thrive on the leaves of plants and hardscapes in the aquarium, and they continue to multiply if favorable conditions exist.

When it comes to the morphological features, both are quite unique— the Green spot algae (GSA) forms tiny, circular green spots on surfaces while the Green dust algae (GDA) appears as a green dust-like film on leaves and aquarium décor.

  • You can tell these two types of algae apart by feeling them with your fingertips, you will notice that Green spot algae are harder/rough when touched while Green dust algae are somewhat smooth to the touch and lift off easily.
  • Another point to note is that while the Green spot algae are tough to scrape off from a surface, Green dust algae can be easily wiped or scrubbed off. However, if left untreated for a long time, Green dust algae will also form a thick layer that can be hard to lift off.
  • Lastly, in terms of growth rate, Green spot algae don’t grow as fast as the Green dust algae.

Causes of Green Spot Algae

This nuisance green algae which are common in freshwater aquariums has been found to be caused by excessive lighting, CO2 deficiency, low or, in some cases, too high phosphate levels. These factors work together to foster the growth of Green spot algae in planted tanks.

Excessive lighting: Planted tanks with excessive lighting and prolonged photoperiods are at higher risks of dealing with serious Green spot algae outbreaks.

Very high lighting in combination with other factors such as low CO2 concentration will result in algae blooms.

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Low nutrient levels: Essentially, an aquarium lacking vital nutrients like phosphates and nitrates is a fertile ground for the formation and growth of Green spot algae.

Therefore, you should ensure that there are sufficient doses of phosphates and other essential nutrients in the tank water which the aquarium plants can use up thereby dwindling the formation of Green spot algae.

For more information, check out:

Are Green Spot Algae Harmful to Plants and Animals?

How to Remove Green Spot Algae in Aquariums on plantsGenerally, Green spot algae are not harmful to animals and, generally, these algae cannot damage the plants, unless the contamination starts getting really bad.

In this case, algae compete with plants for nutrients that can stunt their growth. In addition, when green spot algae cover the plants, the plants cannot perform photosynthesis and grow accordingly. In the worst-case scenario, the plant will simply die.

How to Remove Green Spot Algae

Once you must have identified the cause of the Green spot algae outbreak in your fish tank, here’s how you can get rid of them:

Manual Removal:

How to Remove Green Spot Algae - ManualThis method involves eradicating the green coats from surfaces by scrubbing or scraping them off using special tools. Here are some examples, the choice is huge! (links to Amazon to check the price):

Note: Be careful, if you have an acrylic tank. You need to use tools and special blades that will not be able to scratch the glass of your tank.

This works well for removing them from the aquarium glass and ornaments. Gently scrape the tank walls and ornaments till you completely eliminate all visible traces of Green spot algae. If there are algae coats on the substrate, make sure to scoop or siphon them away together with the detritus.

For plants, the best procedure is to prune the leaves enveloped by Green spot algae and remove them from the tank. Considering how hard these algae can be, it would be practically impossible to wipe them off without damaging the leaves, so just go ahead and cut them off.

What about the live rocks and driftwood? Well, you can take those out of the aquarium and scrub them with a rough sponge, scouring pad, or even an old toothbrush over a running tap.

Chemical Removal:

You can use a chemical solution to kill any remaining pieces that couldn’t be scrubbed off. For example, you can use hydrogen peroxide (cheap and handy household supply that is used for cleaning, healing, hygiene, etc.) or bleach. Then, wash it off properly (twice).

Note: Please, do not forget to use gloves when you are working with hydrogen peroxide or bleach.

Basically, you can use the same methods aquarists use to quarantine and disinfect plants.

Read more about “How to Quarantine and Disinfect Aquarium Plants” here.

Regarding chemicals that can be used in the tank to treat Green spot algae, I would not use any of them. The point is that the cells of these algae are very thick and resistant. Currently, there are no products on the market that can effectively remove these algae in the tank.

There are few reports that products containing algicide might be effective against Green spot algae. Unfortunately, I cannot prove or disprove that.

Introducing Algae Eaters:

Depending on how severe the growth of this algae is in your aquarium, the addition of efficient algae eaters may not be the best option, but it will definitely help in decreasing its population by a significant level.

Good algae eaters for freshwater aquariums include snails and some fish species, for example:

All these species have sucker mouths which they use to anchor to surfaces and scrape algae. 

Dwarf shrimp species (especially Amano shrimp) can also greatly contribute to this battle unless these algae grow on the glass.

Depending on the size of the tank, a sufficient number of these algae eaters need to be present to devour the tough green coats. So, how many snails do you need?

If you are buying them for algae control, the general rule is 1 snail per 5 gallons (~20 liters). Of course, it depends on the conditions of your tank and how many Green spot algae you have. Eventually, you will see the difference, just do not expect fast results.

Phosphate Dosing:

Insufficient amounts of nutrients especially phosphates is one of the major reasons why Green spot algae form in fish tanks.

Lack of nutrition leads to poor plant health and poor plant health promotes algae growth.

Important: Aquatic plants and algae are constantly competing against each other for nutrients. This is called Allelopathy. You can read more about it in my article “Allelopathy or Warfare in Aquarium Plants”.   

Interesting fact: Land plants evolved from their green algal ancestor and it was a pivotal event in the history of life!

Therefore, if you have a Green spot algae problem in your planted tank, then you need to increase the phosphate content in your Estimative Index dosing.

Most aquarists recommend having the phosphate levels in your tank in the 0.05 -1.5 mg/l range. It will boost the plant’s growth keeping the algae at their minimum.

For emphasis, the Estimative Index (EI) is a method for providing nutrients to foster maximum growth conditions in a planted tank. It further entails dosing nutrients to a slight excess throughout a week and performing a 50% water change at the end of the week.

Additionally, you can do a spot treatment with potassium phosphate to combat Green spot algae.

First, endeavor to turn off the tank filter. Next, apply the potassium phosphate solution directly to the affected area.

Similarly, with the E.I in mind, dry potassium phosphate powder (KH2PO4) can be mixed with a little bit of tank water, shake it for a few seconds, and apply onto the affected area using a pipette or syringe. Then, wait for a maximum of 5 minutes and turn on the tank filter.

Keep in mind that too high phosphate levels can be also a problem. It means that there are so many nutrients in the water that your plants cannot consume them all. In these cases, you have to do regular water changes to prevent nutrient buildup.

Important:  If you keep shrimp in the tank, big and (or) frequent water changes, can cause molting problems. So, be careful with that. 

Can We Use UV Sterilizers Against Green Spot Algae?

UV sterilizers can make our life easier in many cases. They effectively reduce the chances of possible algal blooms by making sure that the tank water stays clean and devoid of murkiness. 

The problem is that all UV sterilizers are effective only against free-floating bacteria, algae, and other microorganisms that pass over the bulb.

Unfortunately, Green spot algae do not spread like free-floating organisms, making UV sterilizers absolutely useless against these algae.

In Conclusion

Despite being one of the most prevalent green algae in freshwater aquariums, the Green spot algae are not in any way difficult to curb.

To prevent these nuisance algae, you need to avoid overfeeding and monitor the water parameters closely, make sure to test the phosphate level regularly and keep it at the appropriate level, or else you would be paving the way for a potential outbreak.

Another measure is to decrease the light intensity, if you have low light plants then that would be much easier since it permits you to tone the light intensity all the way down.

Otherwise, you can simply cut back on the photoperiod; keeping the lights on for about 8 hours is enough to support plant growth.

On the other hand, if Green spot algae are existent in your aquarium, proceed to prune the affected shoots, scrape off algae from the tank glass/décor, then combat the problem squarely by dosing sufficient amounts of CO2 and phosphates, and your aquarium will be back to normal in no time.

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2 thoughts on “How to Remove Green Spot Algae in Aquariums

  1. Hi Michael. Thank you for this information. I’ve been trying to lessen grow of green algae that grows all over the glass in all 4 of my fresh water aquariums. I’d been thinking it is green dust, but reading your article makes me think its green spot due to the necessity of scraping it. I haven’t tested for phosphates but will do so. I do very regular scrapings and vacuuming of the substrate to remove waste. One thing that puzzles me is this: I have plants in all the tanks that grow well but I still have the algae. I’ve reduced lighting to 8 hours per day. If the plants are growing well but I have too much algae do I have to much or too little of something? I’m so confused.

    1. Hi Roberta R Rogers,
      You raise a very valid point, but at the same time, it’s a rather complex question.
      In general, plants do have advantages over algae and ideally can suppress their growth. However, there are many different conditions to consider, which we often fail to meet in aquariums.
      For example, it’s not enough to simply have a lot of plants. You need to have a lot of VERY fast-growing plants because rapid growth requires a significant amount of nutrients. The fastest-growing plants are usually floating plants because they have unlimited access to CO2 and brighter light. Submerged plants, on the other hand, also require CO2 injections (again, to enhance growth) and nutrient supplementation, among other things. And that’s just the tip of the iceberg 🙂
      Best regards,

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