8.4 is really not too bad. While a bit on the high side most animals can tolerate abnormal levels of Ph as oppossed to a repid change in Ph. Keep going on the water changes and let the natual buffer on the salt mix do the job. You can also try another test kit for verification. Also, the time of day you test will make a difference. I suggest testing in the morning before the lights come on and again at night right before you shut the lights off. You will likely see a difference with the photoperiod.
Some people are like slinkies...they serve no real purpose yet can still bring a smile to your face when you push them down the stairs!:p
Have a great day! Brian