Fish dying/dead - Heavy Breathing - Need Help

[daniella3d]

is it possible that something was in your tap water? sometimes the water company add chimical, especialy at this time of the year. It is not a good idea use tap water for aquarium use as I have seen it happen and read about it, some chimical killing all the fish in the aquarium. Depending on the dechlorinator, they should be very safe for saltwater. I know Seachem Prime is safe for satwater and I used it before without any problem.

9 fish in a 75 gallons is a bit overcrowded I think. It could have been a factor into your fish death by lack of oxygen maybe.