With PH and Alk, there is an easy rule to remember for all you hot tub owners'. If both PH and Alk are low, add Alk increaser only, test the water an hour later. If Alk is still low add more Alk increaser. Once you get Alk in balance (80 to 120 ppm). Then add PH increaser if needed. If you add both at same time you will get extreme bounce in PH. Alk is a buffer for PH. Always get it in balance, before adjusting PH.
PH down will lower both PH and Alk. If PH is fine, but Alk is high (which is not very common) but can happen, add decreaser, test to make sure Alk is balanced, then you may have to add PH up.
If both PH and Alk are high add PH down. To lower both.