6 gallons of sulfuric acid in 15000 gal pool water over a period of time is not going to increase your sulfates that much. Like I said before, it's about the same as using pH increaser (dry acid), which many people use on a regular basis. Also using non chlorine shock (MPS) adds a lot of sulfates to the water and a lot of people use that on a regular basis (weekly if they are doing it correctly). There is some evidence (depending on who you talk to) that HIGH levels of sulfates in pool water can cause damage to plaster surfaces over time (along with several other causes) but you will not be anywhere near a high level from the amount that you have. Just use it up and when it's gone switch over to muriatic acid (and don't lose any sleep over it!). I would not use it for cleaning your salt cell, however. For that muriatic acid is used.