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If, like me, you like older films like PanF+, FP4+, HP5+ andTri-X you can fix them with Hypo (Sodium Thiosulphate). However, just as with the more modern rapid fixers, if you don't get all the fix out of the emulsion (film or paper) you'll have bad staining a few years down the road. For this reason a Hypo Test was invented. I say good, belt and braces…
Distilled water 1 liter
Potassium Permanganate 1.0g
Sodium Carbonate (anhyd) 1.0g
Place a few mill. of your print or negative wash water in a clear jug and an equal amount of distilled water in another clear jug. Now add one drop of the above formula into each of these jugs and watch how the colour clears from both. It should be at the same rate. If the negative or print wash water clears first there is still residual Hypo in the water. Wash film or paper for longer and retest.
Over used fix also presents major problems. It becomes saturated with development by-products that are difficult to wash out. For this reason never overuse your fixers. Keep a careful tally on how many papers have gone through the paper fix and replace with fresh when the fix has reached its limit, set out by the manufacturer (or in the formula). For film it's easier. When the fix if fresh (first used) make a note of how long it takes to clear the film of milkiness. When it takes twice this time it's time to replace the fix with fresh.
Finally, NEVER use film fix for paper or vice versa. See this post.