Sand will slowly migrate to the bottom and the larger particles will come out on top. Lucky for you there is a literal "flourite black sand" product.
Interesting. I've been looking for a black sand that has the grit size of Tropical Eden's aragonite sand (reef-flakes or grand select). Would this Flourite black sand product be about the same grit size or smaller?
To get an idea, the aragasnow is the grit size of sugar.
(Picture taken from Google)
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It has been about a year since I had flourite black sand, but if I had to pick from those I would say it looks like reef flakes / mini flakes mixed. It does have a fair number of smaller particles mixed in though. Grand select are definitely bigger than it. Much to my annoyance it also has the occasional red particle mixed in too.