Sump question

This is my first saltwater tank, so everything is new to me. I bought a used 90 gallon w/ built in overflow and it also came with a small Berlin sump and Coralife Super Skimmer. I have been cycling for about 4 weeks with the skimmer on. Since I am brand new, I wasn't quite sure how effective my skimming is and I didn't have any instructions since I bought everything used.

Thanks to the Internet, I printed the instructions for the skimmer and I noticed I was missing the bubble-production diffuser . I have noticed some micro bubbles in the tank, but I don't think it is overwhelming. Any suggestions on what I can use or if I need to use a diffuser? Currently the output from the skimmer is just how it is in the picture.

I am including the only picture of my sump and skimmer, which unfortunately was not in use at the time. I actually am not using the filter on the right side (I was told I could put carbon in there), but I would need another pump. Should I also be using this? Another question is should I put bio balls, sponge, ceramic rings, or other kind of filter media in the sump. As you can see, it is a very small sump, but it is rated for 125 gallon. I will eventually upgrade.

Basically, any suggestions or help with filter media and skimmer is appreciated.