That's the billion dollar question most hobbyists wrestle with.
There's a lot more to it than sending in water samples and gaging the results.
I can tell you that the eye opening moment for me was when I turned my attention to pH. My tanks average was well below 8.00. And although I could keep corals alive, I couldn't see much growth.
Now that I maintain ~8.30 24/7, everything turned around.
I'm not saying that's the sole reason for my personal success. There's so much more to it that can't be learned by reading posts, believing marketing and listening to influencers. You have to learn through experience.
Even my oversimplification of "it's pH" can get you in trouble. If someone tells you this is easy, you should squint and tilt your head at everything else they say.