Did you put anything in your water to remove the chlorine and cholramine? Even if you aged your tank water, if your water system has chloramine in it, it will not go away by itself.
How long has the tank been running? If it's not been cycled, your fish could be the victims of an ammonia spike - drowned in their own urine, as it were. Check the ammonia, nitrite, and nitrate levels on the tank. If the first two aren't zero, then don't put any more fish in there until they are. If nitrate is over 40, then that needs to come down too.
Let see, what else could be the problem? Heavy metals - do you have carbon in your filter? has it been changed recently? If not, it may be leaching out all toxins it has absorbed. If that's not the deal, then you may need to get a water treatment that removes heavy metals (AmQuel+ is what I use.)
You don't by chance have any amphibians in that tank?