Put the sand under the substrate, that's what I have done. Originally I put sand on top of Fluval substrate, but after a couple of months the substrate worked its way to the surface so had to hike it all out and sieve it and put back other way round.
Hope you are not snookering the flow of your lovely FX6 in pushing flow through 22/16mm fixings. Area of FX6 pipe is 490sq mm and 22/16 Hydor is a lowly 200sq mm a 60% flow reduction. Your FX6 will not be happy having its flow reduced by so much.
Its says.." Important: Reducing capacity by more than half should be avoided for the following reasons: running noise may increase, the service life of the rotor may be shortened and the cooling required when the pump head is in operation is no longer reliably guaranteed."