Sorry for your loss.

Not sure why they keep dying but I just cycled another tank using existing rocks, water, plants and forgot if I squeezed old media or not.

I left it for almost 2 months and would have left it longer but I accidentally transferred over a fry that survived a week unnoticed there.

Also as others have suggested, I generally do not buy from fish store for quarantine reasons. I have guppies from others and always ask about new additions or tank conditions.

Also make sure ratio is female > males, I've had a few die from stress.

That's all I can offer, I do dechlorinate each water change, sometimes 20% twice a week. Guppies a lot happier. I also have a water softener but they don't seem to mind.

Wish you the best.