Taming a puft appears to increase its kcal needs from ~50kcal/cycle (if I understand the wild puft tooltips correctly) to 200kcal/cycle. The "Glum" state reduces this by 80%, down to 40kcal/cycle. I'm not sure how many kcal this little guy is taking in, but he's in a cycle of starvation and having about 20kcal in the tank. Being tame and happy (normally), this disappears quickly and the puft starts "starving". With 10 cycles left to live if he doesn't eat... he takes in more polluted oxygen a few seconds later, clearing the starvation timer and becoming happy again for another fraction of a minute. The puft next to it is slowly dropping from 1000kcal. It's at about 300 after several (I haven't counted) cycles.
I ruled out oxygen density, I think. At first it was ~800g/m3 (and why is gas pressure measured in terms of mass anyway?), so I pumped in enough to put it as high as 6.9kg/m3. The puft still "starves".
Tame a puft (that means grooming it). Make it happy (just keep grooming it). Now just wait (it may not even need to be happy).