Reef chemistry

Saltwater tanks run on four pillars: salinity, alkalinity (KH), calcium, and magnesium. Corals, coralline algae, and many invertebrates pull these from the water constantly. Stable reef chemistry is non-negotiable — swings cause bleaching, poor growth, and algae outbreaks.

Salinity

Salinity measures dissolved salt content — usually reported as specific gravity (SG) or parts per thousand (ppt).

• Target: 1.025–1.026 SG at 25°C (77°F) for most reef tanks
• Mix saltwater with RO/DI water — tap water adds unwanted nitrate and silicate
• Test with a refractometer (preferred) or calibrated hydrometer
• Evaporation raises salinity — top off with fresh RO only, never saltwater
• Log salinity in Aquarium Tracker alongside other parameters

Alkalinity (KH)

Alkalinity measures carbonate and bicarbonate — the water's ability to buffer pH and supply carbon for coral skeletons.

• Target: 8–12 dKH for most reef tanks (confirm for your coral mix)
• Consumption — stony corals (SPS/LPS) deplete alkalinity fastest
• Low alkalinity — poor coral growth, pH swings, stressed inverts
• High alkalinity — can cause precipitation and tissue damage
• Test 2–3 times per week in actively growing reefs; weekly in fish-only systems

Calcium

Calcium is the primary building block for coral skeletons and coralline algae.

• Target: 400–450 ppm
• Linked to alkalinity — dosing one without monitoring the other causes precipitation (white snow in the tank)
• Sources: Two-part dosing, kalkwasser, calcium reactors, or regular water changes with quality salt mix

Magnesium

Magnesium is often overlooked but essential — it prevents calcium and alkalinity from precipitating out of solution.

• Target: 1250–1350 ppm
• Low magnesium — inability to maintain calcium and alkalinity despite dosing; coralline algae fails
• Test monthly or when calcium/alkalinity behave unpredictably
• Restore with magnesium chloride/sulfate supplements before chasing calcium alone

How the parameters interact

Salinity ──► baseline ocean chemistry
     │
Alkalinity ◄──► Calcium ◄──► Magnesium
     │              │
     └── pH stability └── skeleton growth

Dosing calcium without adequate magnesium wastes product — it precipitates as calcium carbonate. Always test all three together when adjusting.

Testing schedule

Use a master test kit or dedicated reef test kits for accuracy.

Maintaining stability

• Automatic top-off (ATO) — replaces evaporated fresh water; prevents salinity swings
• Protein skimming — exports organics before they break down into nitrate and phosphate; see protein skimmer
• Regular water changes — 10–20% monthly with quality salt mix resets trace elements
• Dosing pumps — for two-part alkalinity and calcium on heavy consumption tanks; see auto doser
• Avoid overfeeding — phosphate fuels algae and stresses coral

Common mistakes

• Topping off with saltwater instead of RO — salinity climbs until fish and coral stress
• Chasing one parameter — always test alk, Ca, and Mg together
• Instant large doses — drip or spread dosing to avoid local precipitation
• Skipping quarantine — marine ich spreads faster than freshwater; see marine ich

Frequently asked questions

If several parameters are off, which do I correct first?

Salinity, then alkalinity. Salinity affects every other reading, so correcting it first prevents you chasing numbers that were never wrong. Alkalinity swings fastest and stresses corals soonest. Calcium and magnesium move slowly enough to address afterwards.

Why does alkalinity fall faster as the tank matures?

Because coral growth consumes it. A young reef with small frags barely moves the number; the same tank two years on can drop measurably each day. Rising consumption is a sign of success, but it means dosing has to scale with the livestock.

Can I dose calcium and alkalinity at the same time?

Not into the same spot at the same moment — they precipitate out as calcium carbonate, so you lose both and cloud the water. Dose them at opposite ends of the tank, or several hours apart, and let flow distribute each before adding the other.

Related guides

• Marine diets
• Marine parasites
• Dinoflagellates
• Bryopsis
• RO & remineralisation