Welcome. Sorry you had to come with such a problem.
They are probably all dying because you used them for cycling. Cycling with fish is very hard on them and most don't survive or have health issues if they do. Let me guess, the pet store told you to go ahead and use them, right? *sigh*
First things first. "Okay" is meaningless as far as water testing goes. Tell them you need specific numbers. "End of the cycle" doesn't mean much either. We need to know specific readings for your
ammonia, nitrites and nitrates. Or you could buy test kits yourself (not the strips, the ones with vials) since you should have them on hand to regularly test your water anyway.
Next, what kind of fish do you have and how many? Did they have any physical signs of illness or disease? Any injuries? White spots, either fuzzy or white looking? Anything else you can tell us will be helpful in diagnosing your problems and figuring out how to avoid them in the future.