Not sure how you interpret the responses to the question as having flummoxed everyone. There are (were) details missing so everything put forth here was speculation.
Umm... because no one answered the questions I asked? You were the only one that didn't talk about sludge at the bottom of the container -- which I didn't mention because that's not what I was asking about. There is no sludge, so talking about sludge makes me think that the question flummoxed the responder. Not to mention the answer about sludge frustrated the questioner!
Speculation as to the crystals/floaties was put forth but like Kodak said, no samples to analyze, no idea what they are. However it is quite common to have a super saturated solution precipitate out "stuff" if it cools down.
I can understand why you'd say that about the stuff in the Kodak bottle. What I'm talking about is a dilution of that -- working film strength rapid fixer without hardener. I don't see how that could possibly be super-saturated. And a super-saturated solution will precipitate out enough to drop to saturated or just below anyway won't it? I remember something I did in a chem. lab thousands of years ago where you tap on a beaker of super-saturated something or other and watch a flurry of precipitation happen.
You had stated a possible maximum temperature (26C) but no minimum. Could also be the temperature cycling that triggered the formation. Dissolved salts from your water supply (didn't see mention of distilled water in the original post but could have missed that tidbit) could have been the seed for the "stuff". No mention in your original post about the history of the jug. Was it brand new? What was in it before? Have you been in the habit of cleaning the jugs between uses?
Temperature would only vary a degree C or so. The fixer sits in a cabinet in a darkroom in a more-or-less temperature controlled house. I did mix with steam distilled water. Don't remember if I mentioned that or not. The jug was not new, but it's only been used for Kodak Rapid Fixer. I do clean the jug between uses, including a final rinse with steam distilled water.
If this was the first use, did you clean out the jug before using it (new does not equal clean)? How about cleaning the lids and the threads at the mouth? Keep your funnel clean (assuming you used one). Do you use proper technique when pouring and don't touch the mouth of the jug to the target vessel (can contaminate the storage container). Etc, etc.
All that, yes. I do all that good clean chemistry stuff. Like not pouring anything back in the jug -- if I pour out too much fixer the extra goes down the drain, not back into the jug.
I echo'ed your use of the word "fragile" because I wasn't sure if you understood the word "oxidize". Since your original post didn't use it I didn't either. But later somebody else introduced you to the word
Now that's just condescending. But if it makes you feel better...
and yes, fixer doesn't seem to oxidize quickly like developers can. I believe however the thiosulfate (soduim / amonium) can turn into a thiosulfite over time and I think that is an oxidation reaction but at room temperature (or at least human tolerable temperatures) it isn't rapid. Chemistry was a long time ago and I just don't remember enough details (wish I did). But all that should have done is render your fixer worthless and you said it still passed the clear-a-film-scrap test. Perhaps Ian or PE or Kirk or Gainer will look in on this thread and pass judgement.
I was hoping that too. But nothing says anyone has to participate. I know I'm a poor writer. If I could write I wouldn't need photography so much! Perhaps if I'd asked the questions differently the problem would have generated more interest and therefore more responses. Sigh. A failing I've had to live with for more decades than I want to think about.
It should store just fine in plastic of the appropriate type. HDPE bottles (#2 or #3 I think, I can never remember the code numbers used for the recycling stamps) should be fine. Kalt should know how to make a plastic photo chemical bottle. Glass would be optimal but it is heavy and fragile once you get to 1/2 and full gallon size jugs. Plastic is much easier to deal with.
Kalt should. But I always seem to have trouble with the plastic bottles, especially the accordion types. And the Vacu-Vin stoppers work so well with wine bottles that I'll probably make that my standard from now on. And glass is so much easier to clean! Those accordion bottles are nearly impossible to get clean inside.
Mix as you go from a concentrate is best. Other than using up my stocks of powdered fixer for some things, I've switched to TF4 and mix from the stock bottle. If you can tolerate a little bit of amonia odor, this is the bee's knees. Or use the TF3 formula posted various places and make your own. Also with the TF4/TF3 you don't use the acidic stop bath, just distilled water so one less thing to mix for each session.
Like I said, this is where I'm heading. Mix as much as I need for a week or two and leave the rest as concentrate in the Kodak bottle. Should solve most if not all of my problems with fixer.
Those things are way more trouble than they are worth. 
