Get the blasting sand, it's just sand, much much cheaper.
As far as a I can tell, when you say red bag, you mean a white bag, with black lettering all over it but a red rectangle on the front and sides. I think! Tractor Supply says they only carry the "red bag" now and that "it's SKU says it's the medium" (I think their SKUs are screwed up).The easiest way to tell between the fine and medium is that the fine is the red bag and the medium is the black. You want the black. It's a coarse sand size. I bought some of the fine when I was setting up my last tank because my local TSC was out of medium. It ended up being a big mistake. There was a gray dust in with it and gave me a cloudy tank for quite awhile.
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