[header] How to treat cyanobacteria (blue green algae) in an aquarium [title] Perform a partial water exchange. [step] If your tank is harboring cyanobacteria, the first step is to replace the nutrient-rich water in your aquarium with clean saltwater. You can do this by performing a partial water exchange. 
[substeps] Aim to remove and replace about 10% to 20% of the tank's water. In a 10 gallon tank, that means about one or two gallons, which should be sufficient to ensure healthy water.