New tank question

[Aries426]

I recently purchased a Fluval Reef M40 AIO tank. I comes with 14g tank, matching stand, Fluval Protein Skimmer, Nano PH, Return and Fluval LED. It has 3 sections in the back like a typical AIO. Looking from the front the left section contains the skimmer, middle is nothing and the right one has the return and heater. I did have the heater in the middle to start with but I found it fits in the right side no problem. My question is what should I put in the middle chamber? It is filled with water to the top. I had a few thoughts-

1. Fuge
2. LR
3. ChemiPure/Purigen

Any thoughts would be helpful.

Thanks