I'll just point out a couple of things.
First, the pH of your tap water can change. This is due to your water source. Some cities have more than one source and depending where they get the water it might fluctuate. Some seasonal changes due to spring rain runoffs and such is also possible.
Second, when testing tap water make sure you outgas the CO2. Water running through pipes can collect CO2 and this will add carbonic acid which obviously lowers the pH. Shaking a small jar of tap water very vigorously for a couple minutes should outgas any CO2; then do the pH test.
This is not necessary for tank water as CO2 does outgas over about 24 hours anyway, as far as the tap issue goes. Obviously CO2 is being regularly and continually produced in an aquarium by respiration of fish, bacteria, plants (if present) but even more by the breakdown of organics. So there can be variance during the day from all this, even without plants. It is always advisable to test pH the same time every day, to get a more consistent and accurate reading.
You mentioned cardinals and gourami. While pH is important, the GH is even more important, especially for cardinals which come from very
soft water. Do you know the GH (and KH or Alkalinity) of the tap water? This you can get from the supply folks, on their website probably.