A plain-English breakdown of every parameter you'll track — what it measures, why it matters, and what to do when it's off.
Alkalinity, calcium, and magnesium are consumed together by corals, coralline algae, and other calcifying organisms. They are closely linked — changing one affects the others.
Alkalinity (also called carbonate hardness or dKH) is the most important of the three. It buffers pH and provides the carbonate that corals use to build their skeletons. Target 8–9 dKH for a stable reef. Swings of more than 0.5 dKH per day stress SPS corals significantly.
Calcium (Ca²⁺) is the mineral backbone of coral skeletons. Target 400–450 ppm. Low calcium slows growth; excess calcium (>500 ppm) precipitates out with magnesium, crashing both.
Magnesium (Mg²⁺) is the forgotten element. It keeps calcium and alkalinity in solution — without adequate magnesium, both will crash even if you're dosing correctly. Target 1250–1350 ppm.
pH measures hydrogen ion concentration — whether water is acidic or basic. Saltwater reef tanks target 8.1–8.4; freshwater community tanks typically target 6.5–7.5 depending on species.
In a reef tank, pH is closely linked to alkalinity and CO₂ levels. Most tanks show a natural daily swing of 0.2–0.3 pH units — lower at night when plants stop photosynthesising, higher in the afternoon at peak photosynthesis. This is normal.
The most common cause of chronically low pH (7.8–8.0) in a reef tank is elevated CO₂ from poor room ventilation. Running an air line from outside the house or using a refugium with a reverse photoperiod both help.
These three are the nitrogen cycle in numbers (see the Nitrogen Cycle guide for the full explanation).
Ammonia (NH₃/NH₄⁺): Must be zero in an established tank. Any reading above 0.25 ppm indicates a problem — dead animal, overstocking, filter failure, or insufficient cycling.
Nitrite (NO₂⁻): Must be zero in an established tank. A reading here means your bacterial colony is overwhelmed or was disrupted (chemical treatments, filter cleaning, new tank).
Nitrate (NO₃⁻): The end product of the nitrogen cycle. Not acutely toxic, but elevated levels stress fish and fuel nuisance algae. Reef: keep below 5–10 ppm for SPS. Fish-only: below 40 ppm. Freshwater planted: 5–20 ppm is acceptable and feeds plants.
Phosphate (PO₄³⁻) is a nutrient that drives algae growth. In a reef tank, target 0.03–0.1 ppm. Too low and corals become pale (nutrient-starved). Too high and coral tissue recedes, zooxanthellae multiply excessively, and nuisance algae take over.
Main phosphate sources: overfeeding, fish waste, tap water (use RODI), and some salt mixes. GFO (granular ferric oxide) media or a refugium with chaeto algae are the most common remediation tools.
Saltwater fish and invertebrates require stable salinity. Natural seawater sits at 35 ppt (1.026 specific gravity). Reef tanks are typically kept at 1.025–1.026.
Evaporation concentrates salt — salinity rises if you don't top off with fresh RODI water. An automatic top-off system (ATO) is one of the most useful investments for a reef tank, maintaining salinity passively.
A calibrated refractometer or a reliable digital salinity probe gives the most accurate readings. Swing-arm hydrometers are notoriously inaccurate and should be retired.
Most reef organisms thrive at 25–27°C. Freshwater tropical species prefer 24–27°C depending on origin. Stable temperature is more important than hitting an exact number — a spike to 30°C or a crash to 22°C is far more damaging than running at a consistent 26°C.
In Singapore's climate, chillers are mandatory for serious reef tanks. Evaporative cooling from fans can drop temperature 2–3°C, which helps during power cuts or as a stop-gap.
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