Green spot algae (GSA)
Green spot-like algae coats

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.

Spot algae

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

Common causes

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)

If you notice an increasing population of spot algae in a planted tank, we recommend you delve a little deeper into the subject of phosphate fertilisation. You'll need a special water test to determine the phosphate content of your aquarium water. Usually, spot algae appear if the phosphate content is too low or too high. If the phosphate reading shows that the level is too low (at the lower detectability threshold) you can easily add this nutrient with the corresponding fertiliser.
However, if the phosphate content is much too high (at around 3 mg/l or above), you'll need to remove some of it by large water changes or by using a phosphate binding filter medium for a bit. If you are adding phosphate with a fertiliser you'll need to reduce the dosage in accordance with the values you measure. Generally, regular water changes are important to prevent nutrient buildup. We recommend you change around 50% of the aquarium water per week.

After adjusting the phosphate content of the water, remove the spot algae from the glass. This will enable you to monitor the formation of new spot algae. Ideally the algae has been stopped, though, or at least is under control.


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.

As a part of your weekly tank maintenance you can clean the inside of your aquarium glass with a blade cleaner and rid it off the green coats easily.