35% Spa Science Shock treatment can be used if your water has fallen from optimal condition form failure to maintain a 100 PPM level. Add a entire 12oz bottle to the Spa and depending on the water you may want to wait and let it do it’s work say like 12 hours then change or flush the filter. If your water has gone bad due to failure in maintaining minimum levels it’s possible a shock treatment will not bring the water back to a desired state.
If water can not be brought back to a desired state empty, clean and refill. Replace the filter as well.
Also using a 35% Spa Science Shock prior to use can have added health benefits creating an exceptionally rich oxygen content in the water.
One measurement in determining the quality of water is dissolve oxygen (DO). Having high levels of oxygen is important in maintaining a healthy happy spa.