Lots of things here so I'll try and keep it organized for you. The first modification I would recommend if you haven't done it already is remove the filter cartridge and its housing pieces, as well as the bioballs from chamber 2. I dislike the bioballs for a variety of reasons but I won't get into that for now.
For skimmers you could look at the Tunze 9002 or AquaticLife Internal Mini Protein Skimmer 115. If you have an Oceanic biocube the AL115 will fit in the 1st chamber. If you have a Coralife biocube it will only fit in chamber 2. In either case if you use the Tunze 9002 it would have to be mounted in chamber 2. I have the Tunze 9002 if you're curious.
For improving flow there are a couple of ways to upgrade both your flow and your filtration. The most basic improvement would be to upgrade the stock return pump. This not only increases the rate at which water flows through your filtration, but it will also provide a slight boost to the flow in the display area. My recommendation for a new pump would be the Cobalt Aquatics MJ1200 but there are also other options. Additionally I would recommend one powerhead to generate flow opposite from your return. What size powerhead you choose may depend on how strong you want the flow to be. I started with a Koralia 425 and it worked well, but if I had to do it again I probably would have gone with a 550 or a 650. If cost is not an issue, the best option (in my opinion, and what I currently use) would be the EcoTech Marine MP10 Vortech.
Copy and paste any of the equipment listed above into a google search for more detailed information and reviews. For alot more filtration options and modifications
www.mediabaskets.com makes alot of cool stuff for many of the major AIO tanks out there, and being based in Wisconsin you could probably save some money on shipping. My last piece of advice would be to really look around, browse YouTube and various forums to see what others have done with their biocubes that has been successful. There are lots of ways to do it, and alot depends on what your specific stocking goals are for the tank, but trust me, there's tons of inspiration out there if you look for it!
