diff --git a/admin_manual/installation/server_tuning.rst b/admin_manual/installation/server_tuning.rst
index 9a639d7ed..12a510ada 100644
--- a/admin_manual/installation/server_tuning.rst
+++ b/admin_manual/installation/server_tuning.rst
@@ -160,6 +160,7 @@ Nextcloud to use Imaginary by editing your `config.php`:
 .. warning::
 
    Make sure to start Imaginary with the `-return-size` command line parameter. Otherwise, there will be a minor performance impact. The flag requires a recent version of Imaginary (newer than v1.2.4) and is by default added to the `aio-imaginary` container.
+   Also make sure to add the capability `SYS_NICE` via `--cap-add=sys_nice` or `cap_add: - SYS_NICE` as it is required by imaginary to generate HEIC previews.
 
 .. note::