I'm doing 100lbs of pool filter sand and 50lbs of 30/60 coal slag in my 72 gallon bow. It's a little bit finer than 20/40, but it's plenty coarse to settle out and not cloud the water. A finer grit will slow down the speed of root-tab nutrients leeching out into the water column. (the finer the grain, the less interstitial-space there is between each particle... the less space there is, the harder it is for water / nutrients to circulate through)
I could only find it in one grade. It is particle ranging from "dust" size up to about a mm in diameter. Rinse for dear life! or be prepared to scrap a lot of filter floss. Plants root and anchor well. No fish problems.
In the tank it almost looks like a mix of mulm and obsidian.