My spa is 450gal. I initially added 2 tblsp of pH down, waited and no change, 2 tblsp more. No change, added 4tblsp before bed (all within a few hours). Since we used the tub for a good hour or so in the evening, I went ahead and put the shock pack in. Checked it 7 hours later, the TA is down to 180 and CH is about 450 (was 240 / 800). PH is still closer to the 8.4 color than 7.8, so I tossed in 6 more tblsp. Hope I'm heading in the right direction.