I absolutely swear by dark brown glass bottles. I get mine from 2 sources. 1 they are cleaned out empty wine bottles .6 of a litre or. 2. half litre screw cap medicine bottles that I buy from my pharmacy for literally next to nothing. I store everything from diluted Ektacolour RA4 replenisher to Ilford Multigrade developer, to the 1st developer used for E6 reversal (Thats the one that goes off )and they literally keeps for years. In both cases I decant one 500cc bottle down into 100cc bottles when I need to break into the next bottle, I find 100cc is just about the right amount for an evenings work.

With the E6 1st developer that will still deteriorate as the level drops, but quite a bit slower than the plastic bottles, so when I take out the quantity I need, I top up the bottle using old glass beads from a cheap broken necklace I bought from a junk shop.