Hello. I am relatively new to film processing, and I am in the process of choosing chemicals for developing film. I need help choosing one of Kodak's fixers. Kodak's website doesn't really seem to be any help because they just list the basic information about their three fixers, Rapid Fixer, Kodak Fixer, and Kodafix 200. Does anyone know the differences between these fixers? Does anyone know any details such as how long they last, how they are meant to be mixed/used... etc. Anything would be helpful. Also could someone explain hardening and non-hardening fixers? I get the basic idea that they keep the film from scratching as easily, but I would like to know all of the advantages and disadvantages of each. Thanks! Also, if anyone includes recommendations of products, please explain why. I like to know why so I can make my own judgements.
Welcome to APUG.
Some of the most entertaining threads over the years have been about fixers

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Hardening helps prevent physical damage to films and papers. Most modern, first-line materials (both film and paper) are very resistant to physical damage. Some of the more "exotic" manufacturers (like the now defunct Efke) made materials that sometimes needed help.
Hardening has some downsides - in particular, with prints, it makes them difficult to tone. For film, it probably doesn't make a lot of difference.
Hardening is useful with some print toning, but only after the development, initial fix and then toning steps are complete.
I prefer fixer that lets me choose to either include hardener, or omit it. The Kodak Rapid Fixer is packaged that way - two solutions, and you can elect to leave the hardener (Part B) out of the mix. I then use that Part B to harden toned prints later.
The other two versions of Kodak fixer are slightly confusing, because they are labelled and marketed under slightly different names in different markets.
Most likely, the Kodak Fixer is the powdered type. That means it is not a rapid fixer, and will take more time to fix your film and paper. It is also a hardening fixer. It does, however have the clear advantage of being a powder, which makes it easier to ship.
I would guess that the fixer you refer to as Kodafix 200 is the same as fixer that I know as liquid Kodafix. It appears to be essentially pre-mixed Kodak Rapid fix, with hardener included. Given its package size, it is probably well suited to small darkroom users.
Hope this helps.
EDIT: the Kodak T-Max films require a strong fixer. Some people have difficulty using the powdered Kodak fixer for those films.