Green spot algae (GSA)

Green spot-like algae coats

Green spot algae (GSA) is a common type of algae that can appear in freshwater aquariums, particularly on glass surfaces, leaves, and decorations.

In order to get answers to basic questions all around algae in the aquarium, we recommend you read this article first.

Aquarium keepers tend to call green, flat, tiny algae spots as spot algae. Those algae are attached to the substrate very strongly. They start out as spots, looking a bit like tiny polka dots. Bit by bit, if their environment is favorable, the spots can widen to extensive coats. They are mostly found on the aquarium glass but also on the equipment and on decoration and plant leaves. Especially the long-lived, hardy leaves of all kinds of Anubias species. Spot algae are frequently found in strongly lit tanks. They probably mostly belong to the green algae of the genus Coleochaete, which have a plate-like multicellular "body".

It is not always easy to discern whether you have spot algae or other green algae coats. The latter also have a tendency to form flat coats and hold on to the substrate quite fast, too. However, these green algae coats do not form the pronounced spots in their early stages. Surely, there are many algae species that have this growth habit. Some green coats can even be wiped off manually very easily, whereas spot algae are always hard to remove. You'll need a blade cleaner for this work, with which you scrape the algae off the glass.

Leaves of an aquatic plant (Bucephalandra) with a pronounced spot algae infestation.

Causes of Green Spot Algae

1. Light Levels: GSA often thrives in aquariums with excessive light exposure. High-intensity lighting over extended periods can encourage its growth.

2. Nutrient Imbalance: High levels of phosphate and nitrate in the water can also contribute to the growth of GSA. These nutrients often come from fish waste, uneaten food, and decaying plant matter.

   Quite frequently, carbon (CO2) and macronutrient (NPK) deficiencies are the reason for the formation of algae. In the following list you can find the favorable values for the individual nutritional elements:

   • A CO2 content of approximately 20-30 mg/l, measurable by a permanent test with test agent

   • 10 to 25 mg/l of nitrate (NO3)

   • 5 to 10 mg/l of potassium (K)

   • 0.1 to 1 mg/l of phosphate (PO4)

   • >10 mg/l of magnesium (Mg)

3. Low CO2 Levels: In planted tanks, insufficient carbon dioxide can limit plant growth, allowing algae like GSA to thrive.

Management & Control

1. Reduce Light: Adjust the lighting schedule to provide less light per day (typically 6-8 hours is recommended for most aquariums). Ensure that the lights are not too close to the tank surface.

2. Nutrient Control: Regularly test your water for phosphate and nitrate levels. Implement water changes and reduce feeding if necessary to maintain balanced nutrient levels.

3. Increase Plant Growth: Healthy, fast-growing plants can outcompete GSA for nutrients. Consider adding more live plants or enhancing the care for existing plants.

4. Use Algae Scrapers: Manually remove GSA from hard surfaces using an aquarium-safe scraper or pad. This helps keep the visible algae in check while you work on underlying causes.

5. Introduce Grazers: Snails (like Nerite snails) or certain fish species (like ottocinclus or amano shrimp) can help keep GSA under control as they feed on it.

6. CO2 Injection: If you have a heavily planted tank, consider implementing a CO2 injection system to help plants grow stronger and outcompete the algae.

Prevention

• Keep a regular maintenance schedule, including water changes and substrate cleaning.

• Monitor feeding practices to avoid overfeeding.

• Perform routine tests of water parameters to catch any issues before they lead to algae blooms.

• Using a sufficient number of algae eaters is an important factor in the field of spot algae control. The most efficient algae-eaters, in our experience, are sun snails (Clithon sp.) or nerite snails (Neritina sp.). They rasp off the hard coats very thoroughly.

By managing light, nutrients, and maintaining a healthy ecosystem, you can effectively control green spot algae in your aquarium.