Well the built in filter is all in how you use it. Foam, ceramic rings, carbon bags. All this is very basic filtration. Although these units are designed to be integrated and used with the filtration provided or as your witnessed water level fluctuations will occur. I would recommend doing the following. Remove any foam blocks, ceramic rings and carbon bags from your filter. Also disconnect and return the Fluval to your LFS
if you can, you shouldn't need it.
Replace the carbon, foam and ceramic rings with live rock rubble and cheatomorpha macro algae. JBJ makes a great clamp on refuge light called a Nano-Grow - pick one of those up, they are pretty cheap. Run that clamped over the live rock rubble and macro algae 24/7. If you want I would recommend adding Purigen by SeaChem and PhosBan by Two Little Fishies, you could add some ChemiPure pro if you like also.
I would attempt to return the Bak Pak skimmer and get a Tunze Nano DOC
9001. They are easy to install and super efficient, should be about the same cost as your Bak Pak as well.