Fixer does have a life after mixing, but it is many months, so it tends to not be a problem. To find out whether fixer is OK, when you do your first roll, drop the cut off leader from the film into some fixer, time how long it takes to clear. Write that time on the bottle. You fix your film for at least twice that time. Every few rolls test the clearing time again. Fix based on this new time. When the clearing time is double what it was with fresh fixer, dump it and mix new.(posted 8694 days ago)If you would tell us what chemistry you bought, we could give more detailed answers as to mixig small batches. But at someone else said, if the chemistry is in powder form you MUST mix the entire amount, making a stock solution, that you then dilute for use.