The sanding and especially the acid wash would produce a lot of plaster dust and would dissolve the plaster into the water as calcium carbonate which would raise the pH quite a bit as well as increase the Calcium Hardness (CH) and Total Alkalinity (TA) somewhat.

I agree with Janet that you just need to add some acid and using Trichlor would be an easy way to do this since you need to increase your CYA level anyway. Initially, you'll need a lot of tablets since they are slow to dissolve and the chlorine should get broken down by sunlight quickly due to no/low CYA, but soon that will change and a smaller number of tablets should suffice.

Richard