Depends on what you want to know

Say someone like Kev needs to know magnesium. It's important to how he runs his tank.
And look at his tank.
It's GORGEOUS


Mine will never look like his.
For me it's alk and ph.
That's what my tank seems to care about.
Personally, I'd say nitrate #1,then phos and alk depending on what corals you are keeping.
But I think the most important tester for all reef tanks is a TDS meter for your RO system.
I honestly believe so many people have so many issues using bad water
Don't know if that helps Amanda.