I need help, betta sinking to bottom

[whoknows]

so my sister has had this betta fish for a while maybe 3ish years and is absolutely awful when it comes to taking care of her fish as she forgets to feed him, and NEVER cleans his tank, as I have many animals of my own I help her on occasion knowing that if I don't do it won't be done but more recently I have noticed that he is sinking to the bottom and won't swim at all, which doesn't seem normal so I have made her give me her fish so I can try to help him and have been keeping his tank nice and clean but I feel like there is something wrong such as fin rot maybe or swim bladder but I have no idea and also no idea how to help this fish I could really use some help for some treatments to try and what I should do about him because I feel bad he has been mistreated by her for so long so please help me get him back t being healthy

I will attach some photos below

Thank You again in advance

 

[oaktree]

Fin rot can be treated with clean water and medicine if deemed necessary.
I believe swim bladder disorder in bettas can be treated with aquarium salt.

This betta looks to be in an unheated, unfiltered bowl. I highly recommend getting him into at least a 2.5 gallon heated/filtered tank if you want any hope of recovery.

 

[rhyan]

Try to move him in larger tank. Filtered and has heater at the mean time while you are preparing his tank just lower his water and do frequent water change.