Why do my fish keep dying

(sorry about the bad photos)

Ok, about four months ago I started a 20 gallon aquarium. Plants sponge filter, HOB. A few weeks after it was cycled I got a dwarf gourami. He died, I accounted it to fragility. I did like a 75% water change.

A few weeks after that I went to my LFS and got five black neon tetras. They were sold to me as easy and hardy. They did a water test for me and said my water was perfect, no ammonia, nitrite, low nitrate. 6.8 ph. One of them died, she was much larger and bloated and I could tell she probably wasn't going to make it. About a week after we got the first group we returned her and got a few more to total a school of 8. Two have died.

I have no idea what I'm doing wrong, if I'm doing anything wrong, the rest are completely asymptomatic, but that's what I said when the first died and the other died the next day.

Please tell me what I'm doing wrong, if anything and how to fix it. Should I got to the store and replace the ones that have died?