WHAT IS THIS

[Austin Wiant]

 

[Austin Wiant]

It's only on one side of my tank and came back after I sucked it up last water change

 

[XNavyDiver]

Hard to tell, and I'm no expert in ID, but it looks like it could be cyano. Scraping the glass clear might make IDing a bit easier.

 

[Crabs McJones]

Possibly cyanobacteria.

 

[Whipples]

Is it stringy and mat-like? Is it releasing air bubbles? If so, most likely cyano. Do you dose anything? Any CuC disappear over the last few days?

If truly cyano, you can try lights out for a few days. Won't harm anything!

 

[LobsterOfJustice]

SAND AND CYANOBACTERIA