Reef chemistry confuses beginners because it gets taught as a wall of acronyms. It's actually four concepts, each of which has a clear purpose.
Salinity (1.023–1.026 SG)
Total dissolved solids. Reef tanks live at ~35 ppt (1.023–1.026 specific gravity). Measure with a calibrated refractometer. Hydrometers drift and lie. Salinity should be your most-stable parameter — top off evaporation with fresh water, never saltwater.
Alkalinity (8–12 dKH)
Buffering capacity. The bicarbonate/carbonate system that keeps pH from swinging when CO₂ enters or leaves. Corals consume alkalinity to build skeletons; if alkalinity drifts low, calcification stalls.
Target: anywhere in the 8–12 dKH range works for most reefs — stability matters more than the exact number. Pick one and hold it.
Calcium (380–450 ppm)
The other half of skeleton-building. Corals deposit calcium carbonate (CaCO₃) — they need both calcium and alkalinity. If one runs out, the other becomes irrelevant.
Magnesium (1300–1400 ppm)
The "supporting actor." Magnesium prevents calcium and alkalinity from precipitating out of solution as they rise. Test it monthly; supplement only when low.
What new keepers get wrong
- Chasing perfect numbers instead of stable numbers. Stability beats target. A tank at alkalinity 7.5 dKH that never moves is healthier than a tank that swings 8.0–10.0 dKH weekly.
- Ignoring magnesium until things go wrong. Low Mg sabotages alkalinity dosing — calcium-alk supplements precipitate instead of staying in solution.
- Testing parameters they aren't dosing. If you don't dose iodine or strontium, don't test for them. The numbers are noise.
Testing cadence
- Weekly while learning: alkalinity, salinity
- Monthly: calcium, magnesium, nitrate, phosphate
- Once tank is stable: alkalinity weekly, everything else monthly
Get a Hanna alkalinity checker. It's worth the investment — easily the single best testing tool in this hobby.
Get the weekly digest
One email a week — new species, fresh guides, ocean news worth your time.