Water chemistry is the single biggest factor separating thriving reef tanks from struggling ones. The good news: you only need to track six parameters consistently, and five of them rarely need intervention once your tank is stable.
Salinity should sit at 1.025 specific gravity (35 ppt). Check it weekly with a refractometer — swing-arm hydrometers drift and lie to you. Temperature stays at 78°F, with no more than 2°F daily variation.
Alkalinity is the parameter most reef keepers obsess over, and for good reason. It drives coral skeletal growth. Target 8-9 dKH and keep it stable — fluctuations of more than 1 dKH per day can cause tissue recession. Calcium should run 420-440 ppm, and magnesium 1280-1350 ppm.
pH tends to follow alkalinity. If your alk is in range and stable, pH usually is too. Nitrate and phosphate are the nutrient pair to watch: aim for 2-5 ppm nitrate and 0.03-0.1 ppm phosphate. Zero is actually bad — corals need some nutrients to thrive.
Test weekly during the first six months. Once you know how your tank behaves, you can drop to bi-weekly or monthly. Consistency matters more than perfection — a stable tank at slightly off numbers beats one yo-yoing through 'ideal' ranges.