So I was basically going to this exact setup and the besta shelf from ikea. Have two questions the board you mounted your apex to do you have room behind this? Also, is the width 22" as the besa is 23 and some change I believe? Also, how long is the skimmer cord? I am going to get that same skimmer and this is the only piece of gear I have yet to purchase and not sure it will reach to the Apex. Did you have to use and extension? If so what one?
There is more than enough room behind the cabinet after I installed the mounting board. See the pick below showing the back. I bet there is 3ft or so. You can mount the board were every you like. The Red Sea skimmer power cord is pretty long. I purchased several of the cords at different lengths that way nothing is real tight. These power cords also have a flat rotating head. Check my post further up and there are a lot of links for all of the stuff I used that may be helpful.