Which bromine cartridges are you using, the purple Caldera ones or the standard yellow ones? If yellow, get the purple ones. They have more bromine than the yellow and for reasons unknown I have always had issues with the yellow ones providing enough bromine, whether it was in a Caldera or Marquis. The Cantabria is a big spa, you may have to keep the bromine cartridge wide open at 6, just due to the size of the Cantabria.