How to control green spot algae (GSA)

There is a difference between green spot algae (GSA) vs green dust algae (GDA). Green spot algae forms distinct circular dots that are very tough to scrape off while green dust algae is easily wiped/scrapped away though it may form a thick layer that is harder to clear. Spots usually take longer to form, while green dust algae can form a green film across the glass within a day or two. Commonly strikes old growth 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.

Having slow growing plants (Anubias, Buceps) in bright light doesn't guarantee that they'll get green spot algae- as long as you can solve the underlying issues in the tank, they can be clean.

Nerite snails do eat GSA & GDA albeit slowly. Shrimps pick on both. Livestock alone are seldom adequate to solve issues though - algae multiples at a much faster rate when the root causes are not solved.

Common Causes

• Poor/stressed plant health (Poor CO2 primarily, or poor nutrient parameters)
• Old damaged leaves
• Plant stress due to unstable tank parameters or poor nutrient parameters
• High organic waste levels if present on hard scape rather than plants

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?