I haven’t been able to get the refills, so I simply purchased 2 each order. 

I have used this salt for many years... I have never seen an issue with pH shift. However much of this depends on your makeup water. Tap water is never recommended for marine use as it varies drastically and has added ingredients that are potentially harmful to all aquatic life ... safe for humans but very inconsistent for marine use. If you are using RO or DI water for your mix then you may need to add some buffers back into the mix to get it to your desired level. The reason for this is ALL salt mixes are designed to use with tap water which can have a high alkalinity level to begin with. Each manufacturer wants their salt to mix clear... so they typically adjust their mix for a worst makeup water condition (tap water) so their water is clear when mixed. The lower alkalinity should not be listed as a negative because aquarist have different requirements and it is easily adjusted with proper addition of buffers. I use and recommend SeaChem Reef Builder (carbonate Alk boost total) & SeaChem Reef Buffer (borate Alk raises pH & total). Their test kit is also recommended as you can test for all 3 Alkalinities (total, carbonate & borate)