I suggest checking Temperature, PH, Alkalinity, Calcium on a regular basis.
Less regular would be Magnesium and Nitrate.
If there is a problem and you are trying to diagnose then test others like Phosphate, Ammonia, Nitrite, Copper, Silicate, etc. These do not need to be tested regularly but can assist when trying to find the cause of an issue.
Chart the results, this is helpful.