A Nature2 cartridge treats only 1000 gallons.  OK for a hot tub but a little 
short for the swimming pool.  I believe they are around $22.  If I wanted to 
treat my swimming pool I would need 8 cartridges.  Two years ago I used 8 
gallons of CS and 1 gallon of CCu in my pool.  Christmas tree customers were 
remaking how clean my pool water was.  I can make CS much cheaper than the 
Nature2 
cartridge but I leave for Arizona after Christmas and trust one of my sons to 
take care of the hot tub.  He sometimes forgets.  It might be better to buy a 
cartridge to use when I am down south, and he could pretty well forget the 
chemicals.
Brickey