Spa startup with a saltwater chlorine generator the water is usually replaced to start with fresh water and then salt is added. For a salt concentration of 2000 PPM that is about 1.5 pounds per 100 gallons of water. Then the water should be balanced which entails at a minimum bringing the pH to 7.5 and the chlorine to 2 PPM.
If the water isn’t balanced with chlorine then a typical problem is the chlorine generator can’t break from a 0 chlorine reading even after a couple days. This type of problem typically only happens at start-up because the fresh water typically has more organic material content and can be high in phosphates leading to a higher chlorine demand than a hot tub chlorine generator can produce. Once the organic material is removed and the phosphates are lowered the chlorine demand of the hot tub water will be reduced and the chlorinator can now make chlorine faster than the spa is consuming it.
Even the salt contributes to the chlorine demand as the salt will most likely have a powdery coating on it unless it was rinsed first. The water should be balanced after the salt has dissolved to also remove any organic material introduced by the salt.
When the water is replaced and after the salt has been added the water should be shocked and a suitable phosphate remover used. If the hot tub chlorine generator will not be on for a couple days then a chlorine based shock should be used. If the chlorinator will be used during start-up a chlorine free shock can be used. When adding phosphate remover simply add a cap-full and if the water gets a white tint to it like a white fog there are phosphate so then add the recommended amount of phosphate remover per the directions on the bottle. After the filter has cleared up the water repeat the process until the water doesn’t turn white when adding the phosphate remover.