Yeah it runs all the time. I did find some anecdotal evidence of problems with water being too clean when running GFO, Carbon and Biopellets. My bio pellet reactor feed directly into the skimmer there by skimming out the anaerobic that consumes the nitrates allowing for more fresh anaerobic to grow. I guess the problem can be that the gfo and carbon, when used in a low bioload situation can consume all the organics in the water column and leave virtually zero nitrates behind to be consumed by the anaerobic. Thus nothing to skim. That's my theory anyway.
On the other hand I read about many people who never had issues. I'll tell you what though, I haven't had to clean the glass in over a week, its crystal.