So I ordered online and picked it up from Tractor Supply. On the Black Diamond Blasting Sandbag it was marked 16/30 and the mark also goes into 12/40. Is this going to be too fine for my tank? I don't want to open it up and check because I won't be able to return it. Has anyone used this grit size? I bought it as medium grit so I am assuming 16/30 and 12/40 is a medium size. What do you all think?