My tap water has a PH of 7.4, but after it sits for a while the PH drops to around 6.2. I tested this theory by putting my tap water in a 5 gallon bucket and let it sit for 24 hours and the PH was the same as the PH in my tank which is 6.2. So I know it was nothing in my tank making the Ph drop. My current fish don't seem to mind the PH being that low but when I try to add fish they cant seem to take it, even after acclimating for an hour.
The tank has been set up for well over a year and this problem just has me baffled. What could be the possible cause of this problem and how could I fix it.