How big is the tank? ( gallon wise / size)
What exactly are the water parameters? ( ammonia, nitrite, nitrate, salinity, temp) please test and give results
Also have you been 'toping off ' with dechlorinated freshwater?
Did you quarantine them in a seperate tank before adding them to the main?
Also when did you add each individual fish ( one a week, all together etc?) No spots ( red blotches, white or black spots, or a gold dusty covering? parasites?) or other weird things showing up on them?
As for the water changes - Its recommended to change 10-20% of the water every one to two weeks ( maybe only a little longer) if water tests are ok.
Such a variety of things could cause sudden deaths like that.. knowing these things will certainly help. Sorry for al lthe questions, but it will help us help you! Sorry for your loss btw