Pictorial Planet
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Don't mix your fix!
Only Use paper fix for paper, film fix for film.
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TL;DR Don't use paper fix with your films, don't use your film fix for your paper!
This was common knowledge in the pre-digital darkroom days but seems to have been forgotten in the film renaissance.
It's all about the silver halides and their byproducts. Isn't everything in photography! Film has around 5 times the concentration of silver halides than paper*, and a large proportion of these are unique to film - specifically silver iodide. Because of this, when fixing, the film imparts a lot more dissolved byproducts** to the fix, exhausting it more quickly.
Of these by-products the film's unexposed silver iodide (remember not present in paper) is particularly difficult to remove by the fix. In fact, according to Gevaert's Manual of Photography 5th edition***, fix can only dissolve 1/10 the amount of silver iodide before becoming exhausted. These dissolved halides (not to mention dyes used in film emulsions) will contaminate paper base and stain it reducing any archival quality.
The problem for us photographers is that this might not happen straight away, leaving you with a false sense of security! However, down the line it'll come back to bite you with staining and discolouration of the photograph! Moral of the story:
Always make up two batches of fix, one just for film and one just for paper. Mark the bottles carefully and keep your fix for film separate from your fix for paper. Never fix more than the makers capacity (see the fix instructions) and, if in doubt make up a fresh batch.
Till next time, John
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* Post Development Processing ©Copyright 1998 by Dr. Michael J. Gudzinowicz https://www.photo.net/discuss/threads/why-two-step-fixing-is-a-really-good-thing.93389/
** "All of our papers are coated with emulsions consisting of crystals formed from a mixture of silver chloride and silver bromide (referred to as chloro-bromo emulsions). Our films use emulsions consisting of crystals that are a mixture of silver bromide and silver iodide (iodo-bromo emulsions)." https://www.ilfordphoto.com/faqs/photographic-paper-faqs/
*** 100ml hypo 20% solution can dissolve 6g silver bromide but only 0.6g silver iodide. Gevaert's Manual of Photography 5th edition 1962