Maybe you changed too much water causing the beneficial bacteria to die-off. Now change 50% - 100% of the water. Only use dechlorinated water with the correct temperature and salinity.
Possibilities include 1) changing the temperature too quickly; 2) water salinity not correct; 3) introducing too much chlorine. Let water sit over-night, verify temperature, check salinity.
Salt water tanks are particularly sensitive as far as changing the ratios of chemicals in the water. It's better to slowly change the water so that the levels change gradually.
