There is no way your PH can drop 2 +/- points from water change with your current tap water at 8.4~8.8 you fish would die with sudden dropped of PH so I would say you did the PH test wrong (if you using API test kit then there are 2 bottle for PH, 1 low ph test and 1 high ph test, using the wrong one will cause the test result error), for ammonia spike i would say something went wrong with your Bio filtration also tresscoat or tresszyme has nothing to do with your PH drop or Ammonia control, and Prime is better than API tresscoat, tresszyme on the declo part. Another thing to consider with large water change without aging your water first then there is a change your tap water chlorine kill your good bacteria before dechlorine like prime taking effect and does it job.
