Heather, I do believe that developer "consumes" oxygen, let me explain. I keep my print chemical solutions in plastic fizzy drink bottles. At the end of the printing session, everything gets back to the bottle. Everything is stored at the same place, resulting in same pressure(s). One day later, the developer bottle has shrunk, not the other two. IIRC, stop bath doesn't oxidise, it's acetic acid with a tiny amount of indicator. Fixer has ingredients that can oxidise (sodium sulfite does), but it's bottle volume remains the same. It seems it's the developing agents that have the tendency to react more rapidly with oxygen.
Originally Posted by Akki14
FWIW, using lighter gas (propane - butane mix) seems to prevent this. Of course, initially the lighter mix has low temperature, so pressure inside will increase significantly. I tried loosening the cap, thus equalising the pressure inside and outside the bottle (you can hear the gas escape). If I let it for a day or two (or more) the bottle's volume doesn't change.