Free tool
Reef water parameter targets
"What should my numbers be?" — the reef-standard target for every core parameter, each one cited. Enter your test results for a plain-English read on what's in range and what to do about the rest.And remember: stability beats hitting an exact number.
The reef target chart
| Parameter | Reef target | Why it matters |
|---|---|---|
| Salinity | 33–35 ppt (SG ≈1.0264 at 25 °C) Salinity ⇄ SG converter → | Osmotic balance for every animal — the foundation number. Reef inverts and corals want full natural-seawater strength. |
| Temperature | 76–82 °F (24.5–27.8 °C) — stable | Drives metabolism; heat spikes cause coral bleaching. A steady 80 °F beats a swinging 78 — stability matters more than the exact number. |
| pH | 8.1–8.4 | Affects coral calcification. It rises and falls with CO₂ in the water and the room, so it swings day to night. |
| Alkalinity | 8–11 dKH (SPS often 7–8) — kept STABLE Reef dosing calculator → | The carbonate that builds coral skeletons — and the single most common swing that kills corals. Stability beats the exact number. |
| Calcium | 400–450 ppm Reef dosing calculator → | The other half of coral skeletons, alongside alkalinity. Corals draw both down together. |
| Magnesium | 1300–1400 ppm Reef dosing calculator → | Keeps calcium and alkalinity in solution — when magnesium is low, you simply can't hold the other two. |
| Nitrate (NO₃) | 2–15 ppm — low, but NOT zero | Coral food. Truly zero nitrate starves corals, pales SPS, and invites dinoflagellates — the modern target is low but measurable. |
| Phosphate (PO₄) | 0.03–0.10 ppm — low, but NOT zero | Coral food, like nitrate. Zero phosphate also drives dinoflagellates and pale, receding corals — keep it low, not absent. |
| Ammonia (NH₃) | 0 — undetectable | Highly toxic. In a cycled tank it should read zero; anything above means an uncycled tank, an overloaded filter, or something dead. |
| Nitrite (NO₂) | 0 — undetectable | A middle step of the nitrogen cycle; it should be undetectable once a tank is established. |
An honest chart, not a promise. These are widely-used reef-standard targets, and different tanks (and different corals) thrive at slightly different points — a stable value near the band beats a "perfect" number that swings. Alkalinity, calcium and magnesium especially: hold themsteady. Your own test kit has the final say.
Sources & method
- Salinity: Randy Holmes-Farley — Specific Gravity (35 ppt ≈ SG 1.0264 at 25 °C).
- Alkalinity, calcium, magnesium, temperature, pH: the reef-standard bands used across our own fact-checked coral care guides (Bulk Reef Supply, Top Shelf Aquatics, Tidal Gardens, Saltwater Aquarium Blog).
- Nitrate & phosphate — low but not zero: Bulk Reef Supply — nutrients as food + modern reef consensus (zero starves corals and triggers dinoflagellates).
- Ammonia & nitrite: both should be undetectable in a cycled marine tank.
A free Coralkin tool — no sign-up, nothing saved. It tells you the targets and reads your numbers; your test kit and a slow, steady hand do the rest. We never guess a care number.