Originally Posted by darren1989
Ok but is it safe in fish tank?
You could try putting vinegar on it. What's tank ph?
Apart from the limestone or sandstone (depending if carbonate cement) lifting ph, those rocks should be fairly safe. I'd go for rocks that look fairly fresh and no soil on them.
If there are any sulphides like pyrite on them (I think low risk), I'd skip those rocks. Just more risk of metals like arsenic.