There's a big thread about flukes somewhere here, maybe someone can post it (I'm on a slow connection now). Some people have had to do 3 or 4 consecutive 5-7 day treatments for persistent flukes, with water changes in between. During each treatment, don't skim (I just remove the skimmer cup and let the skimmer pump aerate the water), and remove chemical filtration and carbon as you've been doing.