OK, the pH being tank water is likely accurate and the tap water should be much the same, unless it was freshly drawn tap water in the tank and tested right away. When testing tap water, you need to ensure the dissolved CO2 is out-gassed, or this can affect the result. Let a glass of tap water sit 24 hours, then test pH. This delay is not needed with tank water as the CO2 will have sorted out throough the agitation and 24-hours.
On the nitrate, this is a case where you should look into dealing with the nitrate before the fresh water is added to the tank. We have a few members here who have gone down this road,
@seangee I think, and certainly
@AbbeysDad has...they will see this and explain it better than I could ever begin to.
GH is 18 dGH (equivalent is 322 ppm). This is fairly hard water. If you stay with this, once the nitrate issue is resolved, your fish options include livebearers, some of the rainbowfishes, rift lake cichlids. Given the nitrate, one optionis to use part RO water to reduce nitrate, and that would also reduce GH proportionally. Something to think about once you get advised on the best methods to deal with the nitrate.