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These are hard circular algae spots that take hard scrubbing too remove from the tank glass.
Moderate to high phosphate limited tanks have more problems with green spot algae on plants.
Diatoms also single-cell organisms but are significantly larger and more complicated than cyanobacteria. Most diatoms show very limited mobility and exist primarily as groups of cells growing a strong film over the surface of object.
Note, cyanobacteria can mimic Brown Diatom Algae, but cyanobacteria are readily swept away with little effort while Brown Diatom Algae is not.
Updated 10/18/17 Brown Algae is not considered a true algae as per many biologists, rather a diatom.
However under the 5 Kingdom classification system both Brown Diatoms and more common algae fall into the "grab bag" Kingdom; "Proctotista", although Brown Diatom Algae fall into the division Bacillariophyceae.
It's generally a good sign if that is the only algae that is noticeable in your tank.
Generally simply scraping the glass and rinsing off plastic plants with a 1/25 bleach/water solution (followed by a rinsing with de-chlorinators and water) prior to your water change is all that's needed to control this easy to control.
Brown diatom algae also out compete more desirable green algae in these conditions when light is poor for healthy photosynthesis due to lack adequate amounts of light in the proper PAR.
The yellowish-brown chloroplasts within Brown Diatom algae are what give this “algae” its typical appearance.
Brown Diatoms are found in fresh and saltwater as well as soil.
FORWARD: This Aquarium Answers article (post) is not intended as a complete algae control article, but is intended to address the types of algae that I receive the most questions about which are Brown Diatom Algae, Freshwater Thread/Hair Algae, Black Beard Brush Algae (BBA), and Marine Hair Algae aka Filamentous marine algae (I address other algae as well in this article).
I will add to and update this article as well over time.
As well, my methods may not work as well for some as all it often takes is new/different variable for an algae control method to be less effective.