Bogwood will soften the water, but it's effect is temporary. crushed coral aragonite or any number of things with similar chemical make-up will counteract this. Slate is metamorphic and stable underwater so as said it will not effect chemistry.
As far as the debate:
Slate from the LFS $6000.00 per ton,
Slate from the quarry less than $100.00 per ton
Any questions
Dave