My fishes are dying

This is my second fish that dying this week and I feel so painful for that. I'm tried all the advices that the seller told me. There's something like white fungus on their body and made them can't swim anymore. The seller told me to keep the water at 25C then the fungus will go away. But I don't see any changes yet:(

Anyone can help me please? I'm a newbie to this.🙏