Best substrate for my first refugium?

[Bryon]

I'm setting up my very first refugium. I was wondering what you fine folks would recommend for the substrate and algae. The reason for me deciding to add a refugium was to be able to provide a constant supply of pods for Flutterbug (my green mandarin). Any help and recommendations would be greatly appreciated.

 

[fort384]

I run my sump just bare bottom with plenty of LR rubble, and I use chaeto in the fuge. It seems to work well as my PO4 never goes above about 0.03 ppm and nitrates are always 0, despite feeding rods food 4-5 times a week and target feeding corals with reef chili.

 

[mfinn]

My sumps are bare bottom.
Just macro.