Hello, my betta fish, Shark bait was depressed at one point and throughout my research I discovered that its best to move/replace their plants/rocks/decorations every so often to prevent depression/boredom. Changing their surroundings outside the tank apparently works also. I've also read betta fish like to 'people' watch so if possible place them somewhere they can observe people.
This was solved with me by purchasing a 10g fish tank which I've since put fake and real plants 2 large rocks I've angled to make a cavern and a resting perch. Shark bait now swims around actively and what appears to be happy, however slightly ill (unrelated ) - working on solving this.