If you want to check if your fixer is still good, you can keep track of the total film surface area it has been used on.
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This article covers my 4×5, 120, and occasional 135 B&W film hand processing.
A film clip test can reveal if your developer and fixer are in good working order. If you enjoy shooting B&W films and want to learn how to develop your film (see my article on how to develop B&W film here), you should include learning how to do a film clip...