hi Mexican Hermit
good question, I think I fluked it to be honest, first we put very course gravel (read course as meaning 20mm diameter( in front of the weirs, then the rock and then I put sand in afterwards.
The sand isnt fine DSB size though it is courser than that,. we have also used crushed shell.
I dont know to some the only way to go is ultra fine sand across the whole tank, but I wanted my tank to look more natural so have used many sizes. not sure if I like it myself yet to be honest as it does trap crap.
Like all things though it would be boring to go the tried and tested route, even if I have to eat humble pie in a month or so and try and syphon it all out again.
We'll see.
Craig.