hey BettaMum i know you are sad right now so sorry for your loss. Just want to make sure you be careful to cross-contaminate any equipment with your other betta. And don't get discourage, you know it always a chance to buy sick betta that is why it so many people on the forum who need help :(
Just want to say that it was example for you and other people when you understand that you always have to quarantine new betta before you put it in the main tank . Well i actually keep all my bettas separate because i always petrified that one will get sick and give it to another one.