Today I was in a hydroponics store to special order the Foliage Pro and my attention was diverted to these bags of silica rock, I could not see inside but the outside said "medium" and I assume I could also order the large or small if I wanted.

I searched some posts here, but could only find references to sand.

Is the only difference the size of the particles?

BTW, of all places, they had bales of Pro Mix! And a months-long search on the internet had only pointed to special ordering them from a place several miles away, over an hours drive on the freeway. This store is 3 minutes from my house. Who would've thought?!

Thanks,

Suzie