Sand

[sjolliff]

I lifted a piece of bogwood up and found an area of sand which was a blue/lilac colour?

anyone know the cause of this?

 

[JustKia]

I believe it's cyanobacteria (sometimes called blue-green algae).
It's not an algae though, it's bacteria, and is to do with dead spots...

OR

its probably a bit of hydrogen sulphide, (anaerobic gas) totally harmless if disturbed, you can leave it as it converts nitrates to nitrogen which gasses off

 

[sjolliff]

Thank you,