black mold on sand

[George Steinberger]

i vacuum it up but comes right
whats is it
i have a skimmer and refugium and Sterilizer
for long time (6) months now
can you help

 

[Highgrade]

Cyanno would be my guess. Is your tank only 6 months old?

 

[Sailingeric]

I had that a few times when my tank was new. It went away quickly, just keep sucking it out.