Vermont Slate Not Aquarium Safe?

[celaeno]

i just got my black slate boulders off ebay (item 360156069384). i never heard of slate failing the acid test, but i just put a few drops of api nitrate test bottle #1 solution on a few rocks, and 3 of the 4 had fizzing. almost all the rocks have a 'shiny' texture to them, even when not wet. anyone know what's going on?

 

[AdAndrews]

i just got my vermont slate boulders off ebay (item 360156069384). i never heard of slate failing the acid test, but i just put a few drops of api nitrate test bottle #1 solution on a few rocks, and 3 of the 4 had fizzing. almost all the rocks have a 'shiny' texture to them, even when not wet. anyone know what's going on?

cant be slate then.

you can still use it, it just raises the ph.

 

[celaeno]

here's a pic of the rocks in question:

if they're not black slate, what would you say they are?