from what i have heard, bichirs prefer sand, at times they stick their heads beneath the sand, problems with small rocks there is a tendency that it would stick in their throat, if you want rock pick pebbles that are not small enough to be eaten
I just switched to sand about a week, almost 2 weeks ago.
I used Quickrete Commercial medium sand. Looks great and the plants stay down better. I can't tell that my birchirs like it any better though. They decided to stop eating after the move except for live foods.