I bought 7 Glofish Skirt tetras 3 weeks ago and today was the first time I've changed 25 - 30% water for them in 10G (38L) aquarium. Replaced water was tap water warmed for few hours and treated with API Quick Start as per water change instructions. 10 minutes after the change all 7 tetras lifted to the surface and started swimming there like gasping for air. It's been like this for 2-3 hours. Before the water change they never swam at surface, they were happy and playful. I have water filter, heater, air pump dispersing air in the water. I measured water quality with strips and all 7 parameters look good, better than it was before water change. Please advise what does this behavior mean.
Update: in a couple of more hours all 7 fish are dead. I still do not understand what went wrong. Something poisonous for them was definitely in the water that killed them in few hours, and I do not understand what exactly. Water parameters per Tetra Easy Strips:
Nitrites: 0.5 ppm
pH: 7.2 - 7.6
KH: 80 - 100
I think as it was my first water change could be something poisonous in new plastic tools that I used, like pump tube or bucket. Though I rinsed them with warm tap water, something poisonous still might had been left.