Eventually, the sand and flourite will mix a bit, this is almost impossible to stop in the long term. You can keep it relatively clean though with a bit of effort.
I had a tank were I tried to separate the two. I used a piece of posterboard like material from an arts/crafts store. I dont remember the name of the maker but it was a white plastic sheet that is used for dry erase markers. With white sand it is nearly invisible. Clear would be prefferable, but I never found anything both flexible enough and durable enough to leave in there long term.
If you go to this link:
And look at my bottom 55 gallon, you can make out the line from the plastic divider vaugely, but like I said, it is pretty well hidden.
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