Lost all fish

Question
I have lost all (15) my fish -mostly gupppie, and do not know what could be the reason. I cleaned the aquarium and a few days later my fish started dying. I went to PetSmart and they tested the water and everything was tested correctly. I had over ten fish for over a year and they were fine so I bought new fish, first a couple died, did not find them right away, could that have been a reason for the other fish to die? Also basic rules of cleaning aquarium - 50 gallon.

Answer
Mary Lou,

when you first purchased the fish in the bags, did you when you got home keep the fish in the bags and put them on top of you water in your aqaurium for at least 15 minutes so that the fish can get use to the new water in your aquarium?
if not, then that is the reason. You should always do that first when you buy fish and take them home.

If not the case, then I am very sorry and don't know what it could be.

thanks.
BethE.