Reef Reference
A complete reference to the parameters that matter in a marine aquarium — what they do, why they matter, and what to do when they drift.
| Parameter | Natural Seawater | Reef Aquarium Target | Unit | Importance |
|---|---|---|---|---|
| pH | 8.1 – 8.3 | 8.1 – 8.3 | — | Critical |
| Alkalinity | 6.5 – 8.5 | 7 – 11 dKH | dKH / meq/L | Critical |
| Calcium | ~420 | 380 – 450 ppm | ppm / mg/L | Critical |
| Magnesium | ~1285 | 1250 – 1350 ppm | ppm / mg/L | Critical |
| Nitrate (NO₃) | < 0.5 | 1 – 10 ppm | ppm / mg/L | Important |
| Phosphate (PO₄) | ~0.006 | 0.05 – 0.1 ppm | ppm / mg/L | Important |
| Salinity | 1.025 – 1.026 | 1.025 – 1.026 | SG | Critical |
| Temperature | 24 – 28 °C | 24 – 26 °C | °C | Critical |
| Iodine | ~0.06 | 0.04 – 0.08 ppm | ppm | Beneficial |
| Strontium | ~8 | 5 – 15 ppm | ppm | Beneficial |
Alkalinity (also called carbonate hardness, kH, or dKH) is the measure of carbonate and bicarbonate ions in the water. It acts as the primary pH buffer in a reef aquarium.
Corals and coralline algae consume alkalinity to build their calcium carbonate skeletons. Alkalinity also prevents pH from crashing. It is one of the most rapidly consumed elements in an active reef tank.
Calcium is one of the major ions in seawater and one of the primary building blocks of coral skeletons, shells, and coralline algae.
Corals, clams, urchins, and coralline algae all require calcium to build their calcium carbonate structures. In a well-stocked reef, calcium can drop significantly each day.
Magnesium is the third most abundant ion in seawater. It plays a critical role in preventing spontaneous precipitation of calcium carbonate, which keeps calcium and alkalinity stable.
Low magnesium is often the hidden cause of calcium and alkalinity problems. Without adequate magnesium, even correct calcium and alkalinity dosing cannot stabilise tank chemistry.
Phosphate is a nutrient that accumulates from fish waste, uneaten food, and decomposition. While NSW has very low phosphate, reef aquaria benefit from a small amount for coral metabolism.
Phosphate at zero starves corals and causes bleaching. However, excess phosphate fuels nuisance algae growth that competes with corals and inhibits skeleton formation.
Nitrate is the end product of the nitrogen cycle in an aquarium, produced as bacteria break down ammonia and nitrite. Unlike fish-only systems, reef aquaria need some nitrate.
Zooxanthellae (the symbiotic algae inside coral tissue) require nitrogen for photosynthesis. Zero nitrate starves coral tissue and causes bleaching.
Natural seawater contains 82 elements at trace concentrations. The most important for reef aquaria include iodine, strontium, potassium, iron, molybdenum, and manganese.
Trace elements are removed by protein skimming, activated carbon, and the biological processes of corals and invertebrates. Regular replenishment maintains the vibrant colouration and health of reef inhabitants.