hatchet die off

[flushingfish]

Trying to figure this out. Yesterday I placed 6 marble hatchets in my 20g planted community tank. Before I introduced them i did my usual water tests.
ammonia=0
nitrite=0
nitrate=5ppm
Ihad just done a water change 2 days ago 25%.
I acclimated them by floating the bag for about 1/2 hr and then introducing tank water little by little for another 1/2 hr. I placed them in the tank and they schooled up with the other 2 that were in there. They ate and all was fine. Within 3 hrs 3 out of 6 were belly up. As far as I know all the steps were followed. I have nice clean water. So I can't figure it out.

Any ideas?

Tests were done with API kit. No test strips

[Zagz]

Based on what you have said, I would have to assume they were ill or stressed right from the lfs. Sorry for your loss.

[flushingfish]

That's pretty much what I figured. They were looking comfortable last night schooled up nad exploring the tank.