Fish dying/dead - Heavy Breathing - Need Help

[apex82]

Day 3 and tank looks empty... All fish hiding and not eating except for 3 out of 9. Albeit there breathing is better than 2 days ago I would still consider it abnormal and rapid. Breathing before was large wide mouth rapid gasps while today mouth is staying more closed but still seems like excessive breathing. I am not sure what to do at this point... If everything was better should they be back to normal by now? Or does an event like this take a week or so for them to start eating and swimming around again due to the stress?

I just don't know if I should try more things( more water changes, different powerheads, take fish out) or if water is okay now but it just takes longer than 3 days for them to start eating and not hiding....