Cinder block?

[SeaMonster]

Assuming it ihas been soaked and rinsed well, safe in reef tank?

Thanks

 

[BCT182]

The sand used may contain silicates... but its debated wether or not silicates are a problem.

 

[trido]

Should be. Cement, Sand, Lime. You may have silicate issues depending on the type of sand used.

 

[trido]

LOL BCT182 beat me too it.

 

[freetareef]

put some in a bucket and let it sit for a few days then check the ph too.

 

[Herefishyfishy]

Silicates are what Diatoms feed on. There are also potentially aluminum compounds and other stray metals (copper?) besides contaminants picked up in transit. I wouldn't use them unless a Fish only system.