You only want to use the Hot Spring silver cartridge. It is designed for your spa.
hottubdan is right, the grey colored nature2 fits perfectly into the circulation filter 'standpipe' of your tub...use backyardplus.com, I think they sell for $30 a pop and anything over $49 is free shipping, they specialize in Hot Spring parts but also sell the nature 2 cartridges...good luck
