how old is your tank?
was the tank fully cycled correctly?
do you test for chlorine?
how old is your test kit ?
do you shake the bottle before use?
you could be getting a false reading api test kits are good for that .
reading through the thread and seeing how the fish are up top gasping for air
I will assume they are also lethargic
I would aim towards ammonia poisoning , it seems the most logical as how the fish are acting , 50% water change but test water before you add it, could have a trace of chlorine depending where your located.