I figured the reason to empty the fixer trays (or tanks) might be to clean them, though that is never mentioned in the references. In my darkroom I always returned the fixer to storage jugs and cleaned the trays. I discarded the fixer in the first tray before it reached exhaustion. I never could see the logic in discarding everything after five cycles. But it's a moot point now. I use ink. But I miss the wet darkroom. I've worked in various darkrooms for around 40 years, starting with a home hobby darkroom as a teen.