Fishes are quite capable of feeling and expressing happiness and it is seen by their movements. 1-Fishes are active, energetic, and entertaining when they are happy. You can see them. 2-swimming actively up and down all around the aquarium. 3-They eat and sleep properly when they are happy and satisfied. 4-A happy fish will play with its owner and will respond quickly. 5-They interact with other members of the tank actively.