Oh how I would love an Arid! Funds are just a little tight right now. I am getting laid off in a few weeks. I am doing by best just to get it up and running. Having the Arid would have saved me some plumbing too not needing to run a reactor in addition. In reality the Arid isn't really more than a skimmer and reactor would be except this build has kind of been a 'what can I find in the closet' build so far. I really hope I can upgrade and possibly include an arid at some point. Would fit great inside my stand and sump.
It looks like you have a custom king sump. That original spot you placed the drain is where his custom reactor was located. You would either T off or run a separate pump. The bottom right location is where the drain should be located.
I had a prior setup almost in the same configuration with the reactor in that place and style. Either fuge or skimmer section (middle). Doesn't look like they left room for a dedicated fuge area or original customer was using an external skimmer and meant for a fuge.
Pretty sure drain is supposed to go there because there is a built in sock holder. The chamber in front right maybe for that custom reactor. An aquac urchin would fit perfect but I stuck my drop in chiller in there. Reactor will be brs dual hung on the side and skimmer will go in fuge area. I will just use a chaeto basket.