Reef Tank Parameters Chart: Every Target Number in One Place

Every reefkeeping question eventually ends at the same place: "what should my numbers be?" This is the complete answer — every parameter that matters in a reef aquarium, its target range, how often to test it, and the errors that make people chase numbers that were never wrong. Bookmark it; this chart is the reference layer under everything else in the Reef Chemistry Handbook.

The chart

Parameter	Target	Acceptable range	Test frequency
Temperature	77–78°F (25–25.5°C)	76–80°F (24.5–26.5°C)	Continuous (controller)
Salinity	35 ppt / SG 1.0264	34–36 ppt (1.0256–1.0271)	Weekly + every water change
Alkalinity	8–9 dKH	7–11 dKH	1–3× per week (daily for SPS)
Calcium	420 ppm	400–450 ppm	Weekly
Magnesium	1300 ppm	1250–1350 ppm	Every 2 weeks
pH	8.1–8.3	7.8–8.4	Weekly (or continuous probe)
Nitrate (NO3)	5 ppm	2–10 ppm (LPS/softie tanks tolerate to ~20)	Weekly
Phosphate (PO4)	0.03–0.05 ppm	0.02–0.1 ppm	Weekly
Ammonia (NH3)	0	0 (only relevant during cycling/emergencies)	During cycle or after deaths

One principle before the details: stability beats perfection. A tank held steady at 7.5 dKH will outperform a tank oscillating between 8 and 10. Whenever you find a parameter outside its range, correct it slowly — chemistry moved gradually is chemistry corals adapt to.

Temperature: 76–80°F

Reefs run best at 77–78°F. Swings matter more than the setpoint: keep daily variation under 1°F with a quality heater on an external controller, because the failure mode of a stuck heater is a cooked tank. Cheap stick heaters fail closed often enough that the controller is not optional equipment on an expensive reef.

Salinity: 35 ppt, non-negotiable

Natural seawater is 35 ppt, which corresponds to a specific gravity of 1.0264 at 25°C. Hold it there. The two common failure modes are both measurement problems, not husbandry problems:

• Uncalibrated instruments. Calibrate a refractometer with 35 ppt calibration fluid — not RO/DI water, which calibrates the zero point but not the reading you actually care about.
• Evaporation drift. Salt does not evaporate; water does. Without an auto top-off, salinity climbs daily and every manual top-off is a mini salinity swing. Automate top-off before you automate anything else.

Mix new saltwater with a quality reef salt mix to 35 ppt, matched to tank temperature, before it ever touches the display.

Alkalinity: 7–11 dKH

The headline parameter of reefkeeping. Alkalinity (carbonate hardness) is the skeletal building material corals consume daily and the buffer that steadies pH. Target 8–9 dKH, test frequently, and correct at no more than 1 dKH per day. It pairs with calcium in a fixed consumption ratio — about 7 ppm calcium per 1 dKH — which is why balanced dosing methods exist. See alkalinity vs. pH for why this number outranks pH on your testing schedule.

Calcium: 400–450 ppm

The other half of skeleton building. Natural seawater carries about 420 ppm, and test kit precision (±10–20 ppm is typical) means anywhere in the 400–450 band is fine. Calcium moves slowly because the pool is large; weekly testing catches trends early. If calcium keeps climbing while alkalinity falls, your dosing is unbalanced — you are adding more calcium than the corals' consumption ratio can use.

Magnesium: 1250–1350 ppm

Magnesium is the bodyguard of the calcium/alkalinity system. At roughly 1300 ppm it interferes with spontaneous calcium carbonate precipitation, letting you hold alkalinity and calcium at reef levels without them raining out onto pumps and heaters. When magnesium sags below ~1200, alkalinity becomes mysteriously impossible to keep up. Test biweekly; it depletes slowly.

pH: 7.8–8.4, mostly informational

pH swings 0.2–0.4 daily with the CO2 cycle — low at dawn, high at dusk. Persistent readings below 7.9 usually mean CO2-rich indoor air, and the fix is gas exchange (fresh-air skimmer intake, CO2 scrubber, refugium on a reverse light cycle), not buffer dosing. Within 7.8–8.4, spend your energy on alkalinity stability instead.

Nitrate: 2–10 ppm — yes, above zero

The era of chasing zero nitrate is over. Corals need nitrogen; tanks stripped to 0 ppm produce pale, starving corals and invite dinoflagellate outbreaks. Target ~5 ppm for a mixed reef. Above ~15–20 ppm, growth slows and nuisance algae gains ground — work it down gradually using the methods in how to lower nitrates.

Phosphate: 0.02–0.1 ppm

Same story as nitrate at 1/100th the scale. Zero phosphate starves corals (they need it for tissue and energy transfer), while chronically high phosphate (>0.1–0.15 ppm) inhibits calcification and fuels algae. The band is narrow enough that hobby-grade colorimeter readings matter — use a low-range phosphate method, not a coarse color card.

Ammonia: zero, and mostly ignorable

After the cycle completes, a stocked reef converts ammonia to nitrate within hours. Test only when something died, power failed, or you disturbed a deep sand bed. Any detectable ammonia in an established tank is an emergency signal, not a tuning knob.

Water changes: the baseline reset

Before any parameter-specific tool, there is the water change: 10–20% weekly or biweekly with a quality salt mixed to 35 ppt. A consistent water change schedule simultaneously nudges every major and trace element back toward natural seawater values, exports nutrients, and dilutes anything accumulating that you are not testing for. Two rules keep it from becoming a parameter disruption: match the new water's temperature and salinity to the tank before it goes in, and test your freshly mixed salt occasionally — salt batches vary, and a mix that comes up at 12 dKH into an 8 dKH tank creates a swing with every change. If your salt tests far from your targets, either switch brands or shrink and spread out the changes.

Testing without fooling yourself

Half of all "parameter problems" are measurement problems. The habits that separate a useful log from expensive noise:

• Retest before you react. Any reading that would trigger a correction earns a second test first. Reagent kits produce occasional wild results from a mis-measured sample or a skipped shake — never dose off a single surprising number.
• Use exact sample volumes. A 10% short sample is a 10% wrong answer on titration tests. Fill to the line at eye level.
• Date your reagents. Most degrade noticeably within a year of opening. A drifting kit produces slow phantom trends that send you chasing problems your tank does not have.
• Test at the same time of day. Alkalinity dips during the photoperiod, pH swings constantly, and even nutrients cycle daily. Same-hour testing makes week-over-week numbers actually comparable.
• Validate against a reference. A reference standard solution — or an ICP cross-check — once or twice a year tells you whether your kit reads true, and in which direction it leans.

Stability: the meta-parameter

If the chart above had a tenth row, it would be rate of change. Corals are remarkably adaptable to imperfect-but-steady conditions and remarkably intolerant of speed: an alkalinity that sags from 9 to 7 dKH over a month is a note in the logbook, while the same move in a day burns tissue. The practical corollaries: automate the fast-moving parameters (temperature via controller, salinity via auto top-off), dose daily rather than weekly, correct any out-of-range value gradually — 1 dKH per day for alkalinity, 1 ppt per day for salinity, a degree per day for temperature — and be suspicious of any product or habit that moves a number quickly. A reef tank rewards boredom.

Trace elements: test occasionally, dose cautiously

Iodine, potassium, strontium, and the rest of the minor elements are consumed in tiny quantities and replenished by water changes in most tanks. Hobby test kits for them range from mediocre to fictional. The sane approach: run a mail-in ICP test two to four times a year to catch genuine deficiencies or contamination (copper, tin from equipment), and never dose an element you cannot measure.

A testing schedule that actually gets done

• Daily (automated): temperature, and pH if you own a probe.
• 1–3× weekly: alkalinity — with a complete reef test kit covering the rest as needed.
• Weekly: salinity, calcium, nitrate, phosphate. Log everything; trends beat snapshots.
• Biweekly: magnesium.
• Quarterly: ICP panel for traces and contaminants.

Fifteen minutes a week, one logbook, and this chart — that is the entire measurement side of reef chemistry.

FAQ

My numbers are all slightly off the targets. Should I fix everything at once?

No. Correct one parameter at a time, slowest-moving first: magnesium, then calcium, then alkalinity, then nutrients. Simultaneous corrections make it impossible to see what helped, and stacked swings stress corals more than any single off-target value.

Do LPS and soft coral tanks need different numbers?

Same chart, wider tolerance. Softie and LPS tanks forgive nitrate up to ~20 ppm and phosphate at the top of the range, and they care less about pinpoint alkalinity stability. SPS-dominant tanks live at the strict end of every range — especially alkalinity.

Which parameters actually crash tanks?

Salinity, temperature, and alkalinity — in roughly that order of speed. Nutrient problems make tanks ugly over weeks; a heater stuck on or a 5 dKH alkalinity crash does damage in hours. Spend your automation budget there first.