Used some similar 6x6 tiles I got from Lowes. They are tough to cut or shape, it was for me anyway! They have grooves on the back which would help keep them in place if laying on grout, but not so much to cut, chip or pound into a design...
Anyway, your asking about them being safe in an aquarium, I soaked them for a few days in scalding hot water, scrubbed them with a brush, and rinsed. I tried the old Vinegar test with no reaction. I then placed in some treated buckets of water testing for ph, gh
, copper, and the standard Api master kit tests. I saw no difference from when adding them to the water, to when I tested again a week later so I rolled the dice and used them to anchor some baby Java Fern's and 3 months later all is still well! My GBR's have even spawned on the bigger pieces a few times.