Quartz is fine in fish tanks. The issue would be whether or not it has been contaminated with anything that would be toxic. Are you getting this stuff out of your land scaping, or digging it out of the ground ? If it's out of the ground then clean it of really good and it should be fine. If it comes out of your landscape, then I wouldn't use it. The reason is because it could be contaminated with herbicides and pesticides that are dangerous to fish. As long as your sure of the source though it's really up to you to use it.