[House Atreides]

“...The unconventional decision to take them into negative territory was aimed at encouraging growth and avoiding deflation, but it meant banks were charged a fee for parking their reserves with the central bank....”
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The banks would have been better off just keeping their required reserves in the form of actual cash sitting “unused”, but safe, in their vaults. Of course, the German government likely wouldn’t allow that rational strategy.

[Toddsterpatriot]

“This increase in vault cash has, however, remained limited,” the ECB said. “Logistical constraints such as storage capacities or maximum amounts covered by insurance are the most likely limitations on (banks’) holding larger amounts of cash.”

German banks hoarding more cash, ECB says