In order to feed your event fish you will need special food, given to you by your coral decorations and transferred to your Event Food Pot. These event decorations are awarded to you by the Hermit Crab with special orange event tickets, which can be found when your fish are released. Go to the Hermit Crab and tap the orange event ticket tab.
Plant these decorations by dragging them to your reef decoration slots and then tap them to harvest the event food. They will have a green triangle above them when ready to harvest.
The food will be transferred to your event food pot - just tap the event food pot and then drag the food to the event species.