Not necessarily. If using Bromine or Enzymes, yes you will need to start with fresh water. If your spa is NOT using Bromine or Enzymes you may continue to use the water. But if your water is over two months old you should start with fresh water. As water evaporates in your tub it starts to raise the level of dissolved solids that can accumulate over time. These solids are not dangerous, but they make it harder for your chemicals to work properly.