You will definetly need more filtration for 200 gallons. A fx5 pushes about 600g and hour while it is clean and that GPh slowly goes down as the media gets dirty. But even while it is clean it will only turn your tank over 3 times in a hour. Now if this WAS a very large sump with loads of bio-media than you might get away with a 3x turn over rate and a low stock. The trouble with a bare tank is that there has to be places for biological bacteria to colonize to stablize your tank. And in a normal tank they would do this on the gravel/sand/decorations/filters/etc/etc. But having a bare tank with no substrate you have to give them other places to colonize. So there for you have to have alot more bio-media. And there is just not enough room for this amount of media in a single fx5. A well layed out sump would be your best bet. But you could do it with a couple of fx5's if you have to.