Green spot algae: how to identify and treat this pest

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Green Spot Algae (GSA) are a relatively little-known type of aquarium algae that most freshwater systems experience at some point in their life. These algae are pretty easy to identify and treat, but require less manual removal than other algae that you may be more familiar with.

Instead, the chemistry of the tank needs to be understood and modified to make the conditions less favorable for these algae to fully grow. Otherwise, treating greenspot algae is relatively simple and straightforward. It may take a little longer to produce results than other types of algae.

Read on to find out how to identify and treat algae with green spots if this occurs in your aquarium!

What are green spot algae?

Green spot algae are green algae that are difficult to remove from your planted tank and can be completely removed. Not many aquarium algae eaters can keep up with their growth and its harder form, unlike soft-haired algae and blue-green algae (cyanobacteria), can be difficult to remove manually. The problem is often a lack of nutrients in the aquarium, so a more rigorous water change schedule is often not enough.

Green spot algae can quickly cover any surface in the aquarium, such as glass, filters, substrate and decorations, if they get out of hand. The biggest problem with greenspot algae is that they can quickly appear on plant leaves and spread to the rest of that plant as well as other plants. As we’ll discuss later, it’s usually best to remove any plant leaves that show signs of greenspot algae.

When out of control, green spot algae can quickly suffocate growing plants from their necessary light source.

How do you recognize green spot algae?

Green spot algae, also commonly abbreviated as GSA, are a type of green algae named for their circular appearance and are usually seen on aquarium glass, plants, rocks, substrates, and other decorations. These thin spots are usually lighter green and can be difficult to scrape off.

What is the difference between green spotted algae and green dust algae?

Green dust algae (GDA) can be very similar to green spot algae, especially if you don’t know exactly what to look for. Both have the ability to cover all surfaces in the tank, including the aquarium glass, which can cause green water to appear. It also doesn’t help that these two algae are sometimes found together on the same plants and surfaces.

The differences between the two, however, are pretty easy to spot. As mentioned earlier, green spot algae (GSA) appear in thin circular slices. Green dust algae (GDA) tend to create a green film over glass and plants.

Another way to tell them apart is to remove them. Green spot algae can be very difficult to remove from a surface and usually requires scraping off. Green dust algae are much easier to remove and can usually either be wiped or wiped off.

Green dust algae are usually the result of a sudden change in the aquarium, be it fluctuating parameters, a change in equipment, too much light, or the addition of livestock, although they can usually be treated with some water changes and forward stability.

What causes algae with green spots?

Green spot algae are an interesting type of algae, as this type of algae does not thrive in planted tanks with excess nutrients, but thrives more in waters with fewer available nutrients, namely phosphates.

When the nutrient levels in the tank are low, it usually means that the algae are absorbing nutrients faster than the plants in the system. This means that the aquarium plants are weakened due to insufficient nutrient absorption and the algae receive all the nutrients available.

In greenspot algae, most hobbyists have found that a lack of phosphates has resulted in ideal growing conditions. It should also be noted that greenspot algae are sometimes the result of low nitrate levels as well.

Green spot algae can also be caused by low levels of carbon dioxide (CO2) in the water column. In the same way, it can make living plants less efficient and allow algae to outperform these species and thrive.

Other factors such as lighting and water flow can also affect greenspot algae growth. This particular type of algae usually grows more in planted tank systems with intense light and / or prolonged photoperiods. Poor water circulation can also cause debris to build up in certain areas of the aquarium, making it easier for algae to anchor themselves.

How do you get rid of green spot algae?

Green-spotted algae can be difficult to remove from a planted tank because they need to be scraped off with a sharp edge. This makes removing glass, plants, and other surfaces even more difficult. The best way to treat algae with green spots is usually to treat the aquarium water first.

As mentioned earlier, low levels of phosphate are usually the main cause of greenspot algae growth. This means that increasing your phosphate levels will help your plants get more nutrients and algae get less nourishment if they grow enough.

Most hobbyists suggest increasing phosphates to at least 10 ppm to prevent new green spots from growing. However, high levels can also lead to other types of algae, so this increase should be slow and steady.

If your plants are infected with greenspot algae, it is best to safely prune these leaves and remove them from the aquarium. Green spot algae can spread between plants when the leaves touch. So it is best to clean / remove the leaves as soon as possible.

The substrate should be vacuumed on any debris that may fall in the meantime. It is also worth checking the lighting on the tank. A planted tank should have a light whose spectrum is designed for optimal photosynthesis.

A good photoperiod lasts an average of 8 hours; Everything else can cause aquarium plants to underperform, and anything that takes longer can lead to other algae like brown algae or staghorn algae. It should be noted that many low-light plant species do not need such strong light, which can save some algae headaches when all other aquarium parameters are checked.

In particularly severe cases of green spot algae, additional carbon dioxide may also need to be injected. However, if you haven’t had any problems, we recommend trying other solutions first as CO2 systems can be expensive.

Do nerite snails eat algae with green spots?

In general, nerite snails are among the best algae eaters for the freshwater aquarium. These snails need brackish water to reproduce, so populations can be easily controlled. While these slugs help keep your tank algae free, they’re not your best bet for actually getting rid of algae.

Snails can take a long time to fight algae growth. There is usually enough time for the algae to replace themselves after consumption. Snails can also produce a significant amount of waste even if they don’t look like that. Similarly, shrimp like Amano shrimp will pick algae from green spots, but will not be enough to single-handedly control the growth in your aquarium.

When it comes to fish, the Otocinclus catfish is one of the best algae eaters. They do require fairly specific tank conditions, however, and it is possible that some green spots will be too difficult to vacuum.

More options for fish and snail algae eaters can be found here in our 8 must-have species for the aquarium!

Does Excel kill green spot algae?

Some hobbyists like to use Seachem Excel to control algae growth. However, we don’t recommend it either. Excel is a popular aquarium product that acts as an organic carbon source. This can sometimes be used as a substitute for CO2 injections as the carbon is still easily absorbed and used in photosynthetic processes.

While some hobbyists have been lucky enough to use Excel to get rid of their green spot algae, we don’t recommend this solution as it doesn’t address the low nutrient problem that is often the leading cause of green spot algae. Instead, we recommend testing the water parameters and supplementing the specific nutrients that are missing in the aquarium. The addition of nutrients directly provides more control over the functioning of the aquarium and allows the import and export of nutrients to be recorded.


Green spot algae are one of the most common types of algae, but most hobbyists don’t even know they exist! It can sometimes be misidentified as hair algae and can also appear with green dust algae.

Green-spotted algae are a little different from other types of algae, but are generally easy to remove over time. This particular alga thrives in nutrient-poor systems where it can outperform other aquarium plant species. To control this growth, it is best to test the water for low levels and dose these nutrients accordingly. Some fish or snails can be added to keep the tank algae free, but both of them won’t completely remove all of the green stains from your tank!

If you have any questions about green spot algae, hair algae, or any other particularly frustrating species of algae, don’t hesitate to leave a comment below!

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