What are the two most important things to test?

[Nilo S]

Alkalinity and phosphates, both using Hanna checkers. I use diy all-for-reef, so no need to test Ca and Mg frequently (I basically never do), and my nitrates are completely under control.

 

[dreamcatcherr9]

Patience and Finances.

Salinity and PH

100!!!

 

[chaoticreefer]

Every system is different, in my system phosphates are the ones that can and do go crazy while depleting my nitrates. If my phosphates are kept at a suitable range, my nitrates stay in a good range too. So If picking 2 tests, it would be alk and phosphates, in which I do test every few days.

 

[725196]

Alkalinity and salinity. Hard to say because there is, imho, five parameters that need to be tested. (PH, calcium, and phosphates).
But Alk and salt are top two in my book