Slate????

[chemflex]

Can you use roofing slate in a freshwater tank?

 

[diVer]

Probably, as long as you grind down the edges so they aren't sharp.

 

[RYO]

YUP SEEN PEEPS COVER THE BACK OF THEIR TANK IN SLATE AND IT LOOKS WICKED

 

[Dragonslair]

Slate by its nature is inert, but I would not recommend using roofing slate. The reason: Over the years the roofing slate will have picked up a whole load of nasties and it would be ingrained into the surface. No amount of scrubbing or boiling will remove these pollutants. If you want to use slate that's fine, but I would either buy it from a slate mine or an lfs or even a stone masons/quarry.
Some quarry's may even give you a piece(off cut)

 

[CFC]

I use roofing slates in my tanks but not ones from old roofs, i use new slates bought from the builders yard. If you do this make sure it is slate and not a modern replica material.