help needed

i have recently purchased another 5 comet goldfish, when I first put them in the tank they swam around, after 1 day I woke up to find 3 of them lying on their sides, not looking too healthy, so I pulled them from the tank and took the water up to be tested. The pet shop tested the water and said that all the levels were correct (even though I had not treated the water for ph levels). I found this quite odd. I then tested the water myself and found it to be very alkaline. I have since adjusted the ph levels to where they should be, added aquarium salt. So I thought all was good. I added the fish back into the tank but now all of them just sit on the bottom (sadly one did die). I don't understand what is going on.. Can anyone help? It is winter where I live could it be that they are too cold?