20/40 grit Black Diamond blasting media if you can get it.
Some rinsing in an old pillow case a little at a time were it me.
30/60 grit works also, but is smaller grain size and plants seem to hold better in the 20/40 grit for me.
The pillow case idea is a GREAT idea- never thought about that! I just got a 50 lb bag for my 55 gallon and was thinking of how to wash it. If I do use this (and remove the current pool filter sand, still undecided) I'll try this out.
Thanks for asking this question OP, was on my mind as well!