Welcome to our forum.
A while back I was wondering if I changed or even partially changed out the water before the 4 months can I keep using the N2 cartriidge.  This is what the N2 folks said"You will need to replace the Nature 2 whenever the water is drained and refilled.  This is because you loose a good percentage of the silver ions when you drain the spa, and their is not enough in a depleted cartridge tobring it back up to the proper level.  So you will need to change the Nature 2 cartridge."My guess is if you  change out 1/2 the water you will need to change the N2 cartiridge as well.  Nice to know someone has the same problem after 2 months.Rick
...I shocked and didn't use it for 4 days.  On the 4th day it was a LITTLE cloudy so I schocked it with the normal 4 Tbls.  The next day it wes still cloudy and no Cl on the test strip.  I reshocked with 4 tbls and the next day again still a LITTLR cloudy and no Cl on the strips.  I then shocked with 8 Tbls and after 40 hrs it wss clear but again no chloriine on the strips. ... So why is it taking so much chlorine?