Be sure to use Dichlor initially to build up the CYA level (around a week or two) and then switch over to using bleach. You want to add enough chlorine after your soak such that you measure around 1-2 ppm FC just before your next soak. If you do not have an ozonator, then roughly speaking every person-hour of soaking in a hot (104ºF) tub will require 3-1/2 teaspoons of Dichlor or 5 fluid ounces of 6% bleach or 7 teaspoons of non-chlorine shock (43% MPS) to oxidize your bather waste (the non-chlorine shock would only be for Nature2 systems or for bromine spas). If you have an ozonator, then the amount of chlorine you will need will be higher if you use the tub less frequently (say, once a week) or lower if you use it more frequently (say, every day or two). Your twice a week usage is in-between so it's hard to predict what would happen if you have an ozonator. Read the link Jason referred to. Getting the Total Alkalinity (TA) lower and using 50 ppm Borates are important in order to have stable pH.