I'm so sorry about your guppy!
I bought prime...didn't know to stop using Stress Zyme but I will stop. Did a 25% water change this morning. The other fish are fine it seems. Maybe he was already sick

of course, I am not taking a poor dead fish back to the store for a $5 refund! How could anyone keep a dead fish? But I will not be getting any more fish from there. My first two mollies came from the same place, just figured I screwed up when they died. Perhaps I did, perhaps not.
Also, the person who took the serpae tetras is bringing me 3 swordtails in a month or so when they fry are bigger, so I guess I'll wait a while too before replacing my Molly.
So, unless something awful happens, I'm not giving up. Just was very disheartened last night. I literally sat and watched him die.
no, my tank wasn't cycled. Long story in another post but I don't think I could have done differently. We live and learn.
Thank you everyone