I was buying sand and gravel for my aquariums the other day (play sand and 'coral sea') and they had some cool rocks they called 'featherlight'. Pretty sure it's some kinda igneous, likely pumice. So I filled my bathtub with water and vinegar, and the rocks have been floating in there for close to a day now. I'm worried they aren't going to sink. In the case they don't I plan on using them with some mortar to make a 'new' rock that hopefully doesn't float. So I'd like to put this in the display, as they are rather pretty rocks, but they have a somewhat abrasive texture to them, so I may just put them in the sump. Am I being over protective?