Fish Are Mysteriously Dying

[Aquamagic]

Well I set up a 10 gallon tank last Friday with my old/established Whisper EX20 filter. Two days after the set up, I bought 6 cardinal Tetras and they were all doing fine for the first few days, but for the past 3 days, I've seen 3 fish die. 2 of the three fish were floating when they died with mostly all of the neon blue coloration gone. What could be the problem? Could it be because there is a parasite in my tank? or could it be because my biological filter crashed??? The water parameters are
PH:6.2, Ammonia and Nitrite:0ppm

 

[Noahs ark6]

ph might be a tiny bit low but that prob wouldnt cause it. you could have waited a bit longer before you ut 6 fish in. that could have caused the biolocial balance to "tip". That might be the prob