I test and log:
alk 2x per week(salifert),
calcium 1x every 2 weeks( salifert),
magnesium 1x per month(salifert) .
No3 1x every two weeks( salifert or api)
Ph via pinpoint monitor daily or as many times as I glance at it during theday/night. I only log the morning readings but did log to track swing for a while.
Salinity daily via pinpoint monitor
Temperature daily
PO4 daily via hanah colorimeter**
ORP daily via Milwaukee monitor**
** I consider these two personal compulsions ;PO4 is not really needed daily and ORP is probably unnecessary all together for most tanks.
Occasional tests:
Iron, Red Sa
Iodine, Red Sea
Potassium, Red Sea