Hi and welcome!
What media do you have in your filter - if it is just the sponge and noodles this does not stop the finer particles returning to the tank, you could try something finer such as filter wool to keep the water clearer. Goldfish are rather poopy fish and produce a lot of waste, and this finer material will need constant cleaning to stop it clogging.
As far as the tests go, how old is the test kit? You could try testing against some RODI water such as Pure Dew that you can buy at the Warehouse or Countdown, or distilling some water and seeing what readings you get from your test kit.
Your PH sounds rather high, you can get a high range Ph kit to test this, or you can ask if one of the LFS such as Deco City Pets or AquaFever to do a test for you if you take a container of water in to them, I am not sure if they will charge you for this.