SWG in a spa works great. I have a 750 gallon built-in. With SWG consistency is key. I have the ability to run the SWG for a specific amount of time, and what I have found, is I run the SWG @ 100% for for 30 mins with a 97 degree water temp every day, and my chlorine level stays fairly consistent. When we get in the spa, we turn the SWG on to compensate for bather load, and leave it on while we are in the spa. Keeping the temperature a constant is the key.