To manage notifications you should go to your device 'Settings'.

Find 'Apps' in the list.

Tap on 'Permissions'.

Select 'Permissions'.

Find and select 'Fishbox' in the list.

Tap on 'Permanent notification'.

Tap on 'Deny'.

After that you will see a cross in a red circle next to the 'Permanent notifications', which will mean that the notifications for Fishbox are turned off.