Reef Tank Water Parameters: Calcium, Alkalinity, and Salinity Explained

If you came to reef keeping from a freshwater tank, your instincts are going to work against you here. Ammonia, nitrite, and pH are still worth knowing, but they’re not the numbers that make or break a reef. The numbers that actually decide whether your corals grow or slowly recede are calcium, alkalinity, and magnesium — and almost nobody explains why until after a beginner has already lost a few frags finding out the hard way.

I made that mistake myself. My first reef tank had picture-perfect ammonia and nitrate, and I still watched a hammer coral shrink for three weeks before I figured out my alkalinity had drifted low enough to starve it. The tank “looked fine.” It wasn’t.

The Numbers That Actually Matter (Reef 2026)

Parameter	Ideal Range	What It Does	Danger Sign	Test Frequency
Salinity	1.025 – 1.026 SG (35 ppt)	Baseline osmotic stability for everything in the tank	Livestock stress, coral tissue recession	Weekly, and before/after any top-off
Alkalinity (KH)	8 – 9.5 dKH	Buffers pH, supplies carbonate for coral skeleton	Swings faster than you’d expect once corals mature	2-3x weekly on an established reef
Calcium	400 – 450 ppm	Raw material for coral skeleton growth	Slow, stunted growth; won’t crash a tank overnight	Weekly
Magnesium	1250 – 1350 ppm	Keeps calcium and alkalinity from precipitating out	Chasing Ca/Alk without success	Every 2 weeks
Phosphate	Under 0.03 ppm	Nutrient export target	Above this, coral coloration fades and algae wins	Weekly
Nitrate	1 – 10 ppm	Some is fine — reef tanks aren’t zero-nitrate systems	Both “too high” and “near zero” cause problems	Weekly

Why These Four Move Together

Calcium, alkalinity, and magnesium aren’t independent dials — they’re a single balancing act. Corals pull calcium and carbonate out of the water to build their skeletons, which is exactly what drains your alkalinity over time on any tank with real coral growth. Magnesium is the part beginners skip, and it’s the reason: magnesium keeps calcium and alkalinity from binding together and precipitating out of solution before your corals ever get to use them. Let magnesium drop and you can dose calcium and alkalinity all day without your numbers actually holding.

Salinity sits underneath all of it. It’s not glamorous, but a saltwater tank running long-term at the wrong specific gravity puts chronic stress on everything living in it, corals included. Check it with a refractometer, not a swing-arm hydrometer — the cheap hydrometers are notoriously off by enough to matter.

My Testing and Dosing Routine

Weekly, every tank:

• Salinity (refractometer)
• Alkalinity
• Calcium
• Nitrate and phosphate

Every two weeks:

• Magnesium

How I dose: Once you know your tank’s actual consumption rate — how much alkalinity and calcium it eats per day — a two-part dosing routine (or a calcium reactor once you’re stocked heavily with SPS) keeps things far more stable than reactive dosing after a bad test result. Dose in small, consistent amounts on a schedule rather than big corrections after the fact. Alkalinity in particular should never move more than about 1 dKH in a single day — fast swings stress coral tissue even when you’re moving the number in the “right” direction.

Common Mistakes I See (and Made)

• Testing only calcium and ignoring alkalinity. They have to be read together, or neither number tells you anything useful.
• Skipping magnesium entirely. It’s the most commonly ignored of the big three and the most common reason dosing “doesn’t work.”
• Trusting a cheap hydrometer. Get a refractometer and calibrate it with a known-salinity reference fluid.
• Big corrective doses after a bad reading. A large jump in alkalinity or calcium is more stressful to coral tissue than the original imbalance was.
• Forgetting that new saltwater mix isn’t automatically “correct.” Test your salt mix after mixing — brands vary, and batches vary.

Full Reef Parameter Checklist

• Salinity 1.025–1.026 SG, checked with a refractometer
• Alkalinity 8–9.5 dKH, tested 2-3x weekly
• Calcium 400–450 ppm, tested weekly
• Magnesium 1250–1350 ppm, tested every 2 weeks
• Phosphate under 0.03 ppm
• Dosing changes are small and spread out, not reactive corrections
• New saltwater batches tested before use, not assumed correct

Final Verdict

A reef tank doesn’t fail because of one bad number — it fails because calcium, alkalinity, and magnesium quietly drift out of balance with each other while everything still “looks fine.” Test all three together, dose in small consistent amounts, and don’t chase corrections after the fact.

Ready to put this into practice? Use the Aquarium Wizard for a tank-specific starting plan, or read Saltwater Aquarium Basics for Beginners if you’re still deciding whether a reef tank is the right next step. If nuisance algae shows up as your nutrients settle in, Reef Tank Algae Control walks through fixing it at the source.