The volume Nagios monitoring parameters fswarning and fscritical are not editable by the user currently, because their values are hard-coded within the rest api.
In openattic/backend/volumes/restapi.py you will find two views (create and update) in the VolumeViewSet. These two views contain the default fswarning and fscritical parameters. They should be editable by the user. We could keep fswarning = 75 and fscritical = 85 as default parameters if no values are given by the user.
If the fswarning and/or fscritical parameter for an existing volume were modified, Nagios needs to be informed about that.