Im so confused and sad

Read my last post in this community:

"Guppy tail falling off" https://www.reddit.com/r/Aquariums/s/6E7nMK5C0j

Today when i woke up, two of my guppies were dead. The one that had that weird tail eating disease and another. The other one had begun to become lethargic and its tail began fraying but i thought it had more time bc it was nowhere near as bad as the white one. The last guppy in the tank is still alive and i begun the bacterial treatment melafix yesterday and did the 2nd dose today so im hoping he can stay alive. I have no other tank to put him in btw. How can i keep the last one alive and any idea why this disease (seems like fin rot but spreads too fast) is so aggressive and quick? Bc the guppy really started showing symtoms then died in 36 hours and the one i thought was fine started showing symtoms and died in 12-20 hours. Please help. Read the other post for details and pics.