You shouldn't need aquarium salt, but you should be adding minerals to the tank. Head to Drs Foster and Smith, in the fish section, and go to
reverse osmosis. They've got mineral mixes made specifically for people using RO water.
The problem with RO without additives is that there is no minerals in the water. So what happens is that osmosis hurts the fish. Water flows from the area of highest concentration (the tank, where there are no minerals) to the area of lowest concentration (the fish's cells). The fish's body then has to work harder to keep the cells functioning as they become bloated.
Different fish have different hardness requirements due to the fact that, in the wild, they came from water with different hardness.