The action button for grabbing does not harvest fish from a nearby fish farm. You have to tap on the fish farm to interact with it.
Steps to Reproduce
1. Place and stock a fish farm. 2. Wait until it has fish in it. 3. Move directly over the fish farm with no other objects nearby for interaction. 4. Tap the grab button: nothing happens. (Or, if there is a different object within range, the character will move away from the fish farm to interact with the other object.) 5. Tap the fish farm itself: the character harvests a fish. (This would be the expected behavior when tapping the grab button if the fish farm is the closest object.)
This issue has not been confirmed by a developer yet.