I have a 200 litre hexagonal tank with a 2 & 1/2 inch bed and I used 3 bags of argos play sand I think?
Dont know if that helps at all?
Make sure you really wash the sand first though, I just 3/4 filled a bucket with sand, sat it somewhere I didnt mind getting waterlogged and left the hose in it, then all the debris just floats out the top as thewater overflows (sorry if you allready know this)