Which Disease?

[AngryPuppy]

Sorry for asking, but what was it that originally killed the other fish?
Perhaps there is a disease in the tank...this can be removed by putting your betta in a clean, plastic cup, emptying out the entire tank, washing everything well and swishing a little hot water through the tank itself.
DO NOT use tap water, as it contains copper, which is lethal to all fish species.
Instead, boil the kettle and pour a small amount of hot water into there. Slosh it around for a few minutes, then tip it out.
Slowly (to prevent the tank from cracking) rinse with cold water, and then you are fine to put everything back into it...

I do not suggest you use some kind of medication...this can sometimes makes things FAR worse...
I have learned from experience..

Anyway, I hope this information is helpful to you.

 

[Flute]

What substrate do you have in the tank? I've been ready to grab a bottle of itch treatment on several occasions before now, only discover that it's just grains of sand stuck to the fish ^^'
I think itch tends to be smaller spots, and as you mentioned that they were larger grains, that's what got me thinking of substrate.