Rock Question?

[enchanted]

I pulled the "river" rocks out of my tank today and replaced them, but I noticed something odd.

On the bottom side of the rocks that were against the sand there were some black spots. :?

I tested the rocks and boiled each for 30 minutes. Anyone have ideas?

Thanks,
Larry

 

[-cAtFiSh-]

algae??

 

[MissClaire]

^^^^ what he/she said

 

[shrimpster97]

probably