Check nsr_jukebox manual and check what is says under "minimum space".

Hello,

low watermark for a jukebox means nothing else that the available volume size
for all tapes in the jukebox are lower then the value in the field "minimum space"
in the jukebox ressource.
We use since years values like "0m" or "0g" as minimum space in our jukebox
ressources, however it never works, we always get message like "....passed the low water mark" also if there a few recyclable tapes in the library.
So you see you could ignore this messages completly, hope that helps,
Stefan