Ok, so your fish should be a part of a blueprint (rather than just a skeletal mesh actor). Instead of using the Level blueprint (which I would never recommend, as it can get messy and doesn’t allow for interfaces) you should do the fish’s logic in its own blueprint (this is a blueprint instance; if you had 5 fish in your scene you would have 5 fish instances).
Your fish blueprint will have the iTween logic in it. Click on Class Settings to add the iTInterface.
Then compile and in your graph you should be able to find “Event On Tween Update” (and also “Event On Tween Complete” while you will need for the destroy phase) in your context menu.
Now set up your tween in the fish blueprint (get rid of the one in your level blueprint) and make sure “self” is plugged into the targets for OnTweenUpdate and OnTweenComplete. You should have both of these events implemented now, so you can add your custom logic to run every frame and when the tween is over.
The switch on Name is optional, especially when you only have tweens only sending one message to any given blueprint. I just included it because it’s a good habit to get into.