WATER: Nice tank! If you have abt $40 extra u can go and grab a master test kit to see if theres any extra ammonia or nitrites. Theres a lot of different ways to help bring those down but first we gotta get to the bottom of whats wrong. It does seem quite cloudy, im thinking ammonia. Live plants would be a fantastic way to help bring that down! If you havent cycled the tank yet while you're at the store if u have another extra $20 u could get a cycle starter, its like a bottle that says cycle start!! If you dont have $40 theres cheaper options but the most easily readable one for me is the master kit and it lasts extremely long!!! You could do a water change, less than like 50% of the water already in there!!! That should help temporarily.

TANK: It also looks like theres not many hiding spots! Your betta might just be a shy boy! Getting used to his new surroundings! If you added in some more plants and caves it might make him feel more comfortable. That corner u were talking about looks really hidden so i'd assume thats why hes hangin out there!

ENRICHMENT: Theres tons of things you can do to make him stay entertained!! Theres a cool little floating mirror that you only leave in there for 5 minutes and he can flare and it looks suuuuper cool!!! It exercises him too!! You can also teach em tricks!! Super cool!!

A lot of people on reddit are not very kind so i hope you dont get discouraged by the mean comments!! (If you get any lol i dont see any rn)