I agree with John here.

I had a very similar situation two seasons ago when I took over from the pool service company my then fiance used.

I had a TA of 400 and a pH of 6.8 or lower. I immediately bought a fountain for the return and installed it. Within a matter of hours, my pH had climbed to 7.0 and I was adding a little acid every 5 hours or so. I think it took me just shy of a weekend to bring my TA to 120 and pH to 7.6.