i was having PH issues in my 100 gal.. ammonia spiked often and we did massive water changes almost every week, and it never seemed to get better. And there was a bad smell.. almost like sewage that we couldn't get rid of. Found out that my PH was 6.0.
i spent half a day reading and trying to find something to add to bring the PH up. What i ended up doing was adding about 10 lbs of crushed coral to the substrate and letting the tank cycle for 24 hours. And now it's 7.4 and the sewage smell is nearly gone.. and the ammonia problem is gone too.