How to control green spot algae (GSA)

There is a difference between green spot algae (GSA) and green dust algae (GDA). Green spot algae forms distinct circular dots that are very difficult to scrape off, whereas green dust algae is easy to wipe/scrape off, although it can form a thick layer that is harder to remove. Spots usually take longer to form, whereas green dust can form a green film on the glass within a day or two. Older growth is usually affected first.

If this is the only algae present in the planted tank, and you are not dosing phosphates at all, it may indicate plants are not receiving enough phosphates. 

In most other cases, green spot occurs on plants either due to slow growth caused by poor growth parameters (poor availability of CO2, nutrients) or due to adaptation stress (e.g. when the plant is transferred to a new environment). In such scenarios, it is common for GSA to affect only the older leaves, while the newer leaves are free of algae.

Boosting growth parameters such as increasing CO2 availability through CO2 injection or dosing more nutrients to allow the plants to grow faster can solve these issues. Robustly growing plants are naturally algae resistant while unhealthy/stressed plants trigger algae attachment. This is a universally applicable rule in any planted aquarium.

High light exacerbates GSA issues, so it is highly recommended to lessen the amount of light on aquariums that are suffering from GSA. However, if you have good growth parameters and your plants are healthy, even slow growers can be grown in high light tanks. 

Bucephalandra and other slow growers are more vulnerable to GSA. You need good growth parameters to keep them free of algae in high light tanks. This means having adequate amounts of CO2 and nutrients available. Bucephalandra brownie ghost 2011 grown under 200+ umols of PAR above.

Nerite snails do eat GSA & GDA albeit slowly. Shrimps pick on both. 

Plants can easily get GSA when they are moved between tanks or when they are newly planted. Algae will usually attack the older leaves while the plant channels energy into defending new growth. Older, decaying leaves should be cut off. Removing older, algae-infested growth and replanting fresh tops will rejuvenate the tank over time.

Growing slower growing plants such as Anubias in a high light environment makes them more vulnerable to algae. Growing species that have high light requirements beside ones that have low light requirements puts the lower light requirement species at higher risk of getting algae.

Common Causes

• Slow growth due to poor availability of CO2/nutrients
• Plant adaptation stress due to new environment
• Deteriorating older leaves that are not cleared

Solutions

• ​Prune off old/dead growth, replant new tips
• Address plant health issues
• Spot-dose APT Fix
• Clear organic detritus in planted tank

 Here are links for further reading on the following topics:

1. Section on water parameters

2. What nutrients do aquarium plants need?