Mitsufishi makes a good point. Did you clean the tank after the first fish passed before adding the new ones?
Its very possible that there may be something in your water that is bad for the fish that isn't even tested on those strips. Do you have down water or well water?
I would wash the whole tank, decorations everything with a bleach solution (9 parts water 1 part bleach) then rinse with HOT water. The wash again with vinegar, rinse again with HOT water and then let air dry for a couple days.
Then try again, I would try maybe using some RO water or something, or find another source of water besides wherever you get your water from and see if it makes a difference!