If you are using RO water, you don't need dechlorination.
That is what RO water is, It is tap water put thru a process to remove everything that is added to the water by the cities. Chlorine, floride, copper, iron, chemical to kill the bacteria. Just put in the RO water.
You don't have you sand or live rock in the tank yet do you. If you do, then you should mix up your saltwater in a separate container. Never mix salt in your tank, It could be davastating.
But if your tank is empty just water in it, it should be ok. Just put power heads in the tank to get things mixed up properly.
But in the future, mix your saltwater in a separate container, let it age about 24 hours before you add it to the tank. Keep a powerhead for the mixing container, and a heater. Before you do your water changes, try to get the new water as close to the old water as you can, temp, salinity, ph, that way you don't shock your fish.