Its every thing that you have to look at and its usually something obvious that we simply ignored. Start with your water. Where do you get it from or if you make your ro/di water and how do you check your filters for chlorine and tds. then the water in the tank. We are all stuck testing the big three, nitrates, phosphates and alk. If your alk is stable almost everything else will be because it is so much work to test and observe alk levels. So it then boils down to how you keep your alk stable: manual, doser, or ca reactor. Salinity, refractometers need to be calibrated to make sure you are not having fluctuations or are simply out of whack. And how do you keep your salinity stable, do you use an ATO or do you manual add ro/di water for evaporation. And triton testing is another means to validate and determine if there are bizarre levels of many things like trace elements or heavy metals. Water changes have their pros and cons but I am a pro water changer (10-15 percent weekly or so)
then there is temperature control. pH stability. Flow. Lighting. Filtration, skimmer, gfo, carbon with pros and cons to each. Its late at night so I will leave it to others to give you more things to consider and look at. None of it is hard, but it is a large laundry list of to do items to check and understand your tank.
