Air is the enemy
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Why developer goes off
The developing agents are reducing agents, and they react with oxygen as readily as they react with exposed silver halide. A bottle with air in the headspace is slowly consuming its own developer.
Which makes headspace the variable
Everywhere else in this library headspace is something to leave for expansion or for remixing. Here it is the thing to eliminate, and a bottle filled to the neck keeps far better than one half full.
So buy small and decant
Splitting a liter of stock developer into several small bottles, each filled full and opened only when needed, is the standard practice. The shelf supports it: 132 of the 191 containers are at or under 500 mL.
And the symptom is subtle
Partially oxidized developer does not look wrong. It develops more slowly and to a lower contrast, which reads as a bad film or a bad exposure rather than as a packaging failure. That misattribution is the real cost.
Collapsible containers exist for this
And we do not stock them. A concertina bottle that collapses as it empties keeps the air out without decanting, and it is a format worth knowing about even though it is not on this shelf.
Light matters less than people assume
Most darkroom chemistry is not especially light sensitive in a closed bottle, and the 30 containers here that exclude light are a reasonable option rather than a requirement. Air is the first-order problem.
Fixer is more tolerant
It does not oxidize the way developer does, so a larger part-full bottle of fixer is less of a problem. The two chemistries want different container strategies, which is worth knowing before standardizing on one.