Some of these have been covered in the past and for some these may seem to basic but I thought I would through some ideas out there to get the conversation started.
Lighting: pro's and con's of the different types.
Sump/Refugium set-ups and best practices
Methods for keeping Phosphates in check
As Apex continues to gain in popularity; programming your APEX to get the most out of it
Water flow in a reef aquarium
Using a calcium reactor; pro's and con's and how to set them up correctly
Hardware Update to simplify keeping a marine aquarium; automation, ATO's, other system outside of APEX, water testing, leak detection, auto water changes, etc.