Awww...so sorry..very sad:(
I am wondering why they all getting sick. Do not share anything between sick fish and other fish. I have 10 bettas and one of them 5 years old and the youngest one about 2 years old. Even though they don't get sick i still trying to use separate changing cup for all of them. Even when i feed them if i touch the water in one tank i rinse my hands before i go to feed another one. And all my bettas in separate tanks too.
oo.. sorry just tried to search for you previous post and saw your comment
Something got into my sorority (& males' tank by cross-contamination)....
Do you still any betta left:( Are they all sick? Do you think any of them will pull through? Just feel so bad:(
Don't get discourage you did good thing that you rescued them from the store , at least they were loved:( I used to rescued sick betta before .
But i am sure you can just disinfect everything and start over...Do you need disinfection instructions?
Don't give up.
Also don't make water too warm it will cause bacteria to flourish