Surprised you have not had more responses to your post.
The magnum 350s move alot of water and work fine for mechanical waste. But there design is that you chnage the sleeve and the carbon on a regular basis. This gives you no place to grow a beneficial bacteria colony, which removes the harmful ammonia and nitrites from you aquarium. One thing you could do is fill the media container with biomax, this will give you an area to grow the bacteria. I have found that the bioimax does become dirty quickly requiring frequent cleaning.
If you feel that you need to skim the surface of you water to remove debries that is fine. Either way you need to aggitate the waters surface for good oxygen exchange. I would search in the DIY section to find ideas to built a spray bar to hook to one of your magnums. Also you will find that after aggitating the surface you may not need the skimmer.