Assuming you floated the new ones and introduced your tank water slowly. My experience has been that fish just sometimes die from the transition. Might have been stressed already in the store.
Also, I often notice that new fish will behave erratically in the new environment for a couple of days while they calm down and begin to feel safe.
One last thing:. I have gotten panicky in the past and added things/done things just hoping they would work etc. If your water is right (and it seems like it is), I would do nothing and see what happens.
Also, your fish look good to me and your tank is pretty