Please help!!!!!! What is this??

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This started appearing in my daughters tank on the sand it looks







purple with black hairy stuff on the rocks it is long black hairy looking, does anyone know what it is and how we get rid of it????

 

[mr_X]

Cyanobacteria. Water changes, vacuuming out what you can with each change, and cutting back on feeding. It's from excess nutrients.

 

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Thank you