jetajockey
come get me tang police!
Thanks everybody for your advices.
Have been searching about the cause of mass of death of my new fishes. Found one articles explaining some reasons for this symptom in tetras (neon, cardinal, rednose, etc). The reason was:
1. Acclimating
2. Tank is still cycling
3. Difference of pH value
4. Fast selling cycle in the shop (when the fish was not fully acclimated to shop's water, the fish was sold to the buyer)
For those reason, I tought a reasonable reason for my case is number 3 or 4. But number 3 should be OK with acclimating..... so number 4 ?
I don't buy the 'collision in the bag' LFS response, kinda silly IMO. As long as they weren't constantly being freaked out during the acclimation period it shouldn't be a problem.
Not to mention, the LFS should know better since they receive their fish in densely packed bags themselves and those stay in shipping overnight or even a few days, so that argument doesn't work at all.
The pH acclimation thing is kind of a myth also, but even still, a 3 hour acclimation is more than enough for even a huge pH shift, and that is a relatively small one.
#4 is possible, if your water tests all look fine then I would point more into the direction of a stock issue from the LFS or possible aggression in the tank (sorry don't remember your exact stock).