3 peacock cichlids dead within 3 days?

As the title says 3 peacocks dead each one day. 

 I have a 60-gallon tank, 2 60 gallon sponge filters with aerators and an electric heater thats set to 78-80.

2 of my peacocks did a swim to the top popped out the water a little and wound up on their sides to die inside of the tank.

The other one died when I wasn't watching it today. I tried feeding them bloodworms but only 2 of the peacocks ate and only on their first days in the tank.this is because one of the three was near death coming out of the bag lying on its side before releasing it in the tank.

The store I bought them from uses copper in their water and I was told to drain off all that water before pouring it into my tank which i did. 

I tried doing a water change gravel vac and adding some aquarium salt and de-chlorinator and the last fish still died... Please note that the one that died today only hung out at the bottom of the tank hiding.

is there something going on with the copper water to normal water transfer cuz im like wtf. I've never had 3 fish die on me this fast. I'm shocked because the color they had in the store was much duller before coming into my tank... maybe it was just the black gravel substrate. 

I'm going to get a water check later today at my LPS and see what I come up with because I don't want this to keep happening if it's my water. Any ideas as to what might be going on in my tank?