there are a lot of things you can do to the biocube. I have a 14g and have removed the bioballs(trap nitrates) and added some denitrate cause i had a nitrate problem. I put some filter floss on top of the drip tray and replace the brown floss every week or so, but i had to cut the overflow to the drip tray to increase enough flow. Also took out the sponge/carbon filter in the first chamber and put in a regular sponge. Lastly took out the sponge in the third chamber and put some carbon in its place (since i took it out of the first chamber).
Some people make a refugium out of the bioball chamber. They scrap the paint off the rear of the tank where that chamber is and put a small PC light behind it and then put some chaeto and some lr rubble in it.
Oh yeah i also have an extra power head in the actuall tank part. but some people put them in one of the rear chambers and drill a hole in the back to leave more room.
You could also upgrade the lighting if you wanted clams or sps.
Just look around at the tank diaries and read about there mods (there is one guy out there that only runs carbon and thats it no sponges, or bioballs, or anything, but he has a lot of lr in his tank. Ill try to find the thread.