Hi everyone, (tl;dr: I want to learn about all the different types of fixer and how to tell them apart.) I currently use Zonal Pro (by Alta) Extended Capacity Rapid Fixer. Sadly, this is a company that has shut its doors in the past few months. The bottle says it contains both Ammonium thiosulfate and acetic acid. Now, I know that the ammonium thiosulfateas opposed to sodium thiosulfatemakes this a rapid fixer. But I had also heard somewhere that ammonium fixers were alkaline fixers...? I remember from Steve Anchell's Darkroom Cookbook that alkaline fixers had many advantages over acid fixers. I then read somewhere else that alkaline fixers were simply non-hardening fixers. Basically, my questions are: 1) Is there a way to tellbeyond litmus testingwhether a particular fixer is acidic or alkaline? 2) I'm thinking, based on the advantages listed in TDC, that I would like to try an alkaline fixer. However, all that's mentioned is the Formulary's TF-4. Are there other commercially available alkaline fixers? Do they work for paper processing as well as film? 3) What's the deal with hardeners? Do they change the pH of the fixer? I occasionally want a hardening fixer for films like Efke 25, and it would be nice to just add a secondary step rather than mix up a whole batch of Kodafix, which doesn't last at all. I'll leave it at that for now, since I'm sure all your answers will only raise more questions.