Debris disks are the extrasolar analogues of the asteroid and Kuiper belt. They provide important constraints for planet formation theories. However, the gas content of debris disks is still poorly understood. So far, it has been studied mainly with CO observations. Complementary information can be obtained by observing C. We present a rich ALMA data set (supplemented with archival Herschel data) of 14 debris disks with both CO and C observed. We derive column densities in an uniform way. We compare our results to thermo-chemical models as well as models of gas production from comet evaporation. We find that both classes of models cannot fully explain the data, indicating that they might be missing some key aspect of the nature of debris disk gas. Our data set provides a useful benchmark to constrain current models and advance our understanding of the gas content of debris disks.