Just wanted to post this for some advice on this setup.
I am running a Sapphire Aquatics BC29 Protein Skimmer in my BioCube29. This is a custom made skimmer for this tank.
After some time I have figured out two things. Forgive me if you already know this or if this is old news.
1. Even though the new model skimmer says that there is no modification needed, you will need to cut the back plastic rib on the filter entry cover. This will alow the skimate removal cup to sit at the proper height. (See Pic #1)
2. You WILL have to remove your pre-filter cartridge (the carbon / fiber filter that is in the far right chamber). When this filter cartridge starts to collect particulate, it reduces the flow to the second chamber. Therefore the water level begins to drop. It seems like evaporation at first, but the salinity/SG
do not change. Eventually the flow will get restricted so much that the level drops down too far for the pump and it will run dry. Just add a bag of Carbon in that chamber. You will be able to fit a decent size bag in there without obstructing any flow (See Pic #2). After speaking with the skimmer manufacturer, it was agreed that the skimmer plus a bag of carbon will be sufficient.