You may use the test kits, but they do need calibrated to ensure accuracy, otherwise you have little confidence that they are accurate and telling you "the truth".

This is a more complicated process and you need a decent scale and the chemicals and a way to measure liquids accuracy etc.

A lot more to learn and do
the problem in the hobby, one I've stated going back a de cade or more now, is that folks do not bother, they assume, like most hobbyists do, that the test kits are accurate to begin with, they are not.

I thought so years ago.
Advice says test, so I did.
I had little notion that the test kits might not be accurate.

It's a wake up call to some degree.

Regards,
Tom Barr