Killed My Guppies Somehow

[JaimeShaye]

I had six guppy fry and bought a new tank to move them to once the tank cycled. Well, the tank cycled. I added Fluval cycle and did the API Water Test Kit. Everything was good, so I thought. I took the fry about 3 weeks old by now and moved them to the new tank. Within about two hours they were dead. I don't know what I've done wrong. I'm so upset!
The readings were good. No ammonia, nitrite, PH about 7.4. The heater was working and about 77-ish.
What did I do wrong?

 

[Thefishlady1]

Yes I am no expert but this happened to me with my last 3 fish I got new ones and I am not going to try and move them at all to see if that helps ��

 

[Desperado070]

Please tell me what you think is cycling a tank.
How did you do it? in detail...

 

[SaraB]

How quickly did you add them?