What is this stuff?

[ryn_C_mu]

So I have been getting this dark brown on the live sand ever since I started to feed my inverts before I got my fish I take all uneaten food out and even if I do it comes back so what is it?

 

[Bectan01]

New tank? Probably diatoms - will go away on its own over time

 

[ryn_C_mu]

It's been 4 weeks since my baby started so yah what are diatoms?