Saltwater aquarium reference

Recommended Saltwater Parameters for Reef Tanks

Updated August 2026. For a beginner reef tank, I would aim for 35 ppt salinity, 77–78°F, about 9 dKH alkalinity, 400–450 ppm calcium, 1,250–1,350 ppm magnesium, detectable but controlled nitrate and phosphate, and no measurable ammonia or nitrite after cycling. Those are working targets—not numbers you should chase every time a test moves slightly.

The larger lesson from more than a decade of keeping saltwater aquariums is that stability and trends matter more than a perfect reading. Test the same way each time, write the result down, confirm surprising readings, and make one measured correction at a time.

Written by Matthew Zahler, founder of My First Fish Tank. This revision was checked against the Merck Veterinary Manual water-quality testing table, NOAA coral-system guidance, and the current BRS reef-tank parameter reference.

Disclosure: This guide contains affiliate links. If you buy through them, My First Fish Tank may earn a commission at no extra cost to you.

The Quick Answer: My Working Reef-Tank Ranges

These are the ranges I use as a practical starting point for a mixed beginner reef. The animals you keep, your salt mix, and the instructions for your test method still matter.

Parameter	Practical target range	My usual target	How often I check
Salinity	33–35 ppt	35 ppt	Weekly and before water changes
Specific gravity	About 1.024–1.026	About 1.026	Use one salinity unit consistently
Temperature	75–80°F	77–78°F	Monitor continuously
pH	About 7.8–8.3	A stable value near 8.0–8.3	Trend it; do not chase one reading
Alkalinity	8–10 dKH	About 9 dKH	Weekly; more often while dialing in dosing
Calcium	400–450 ppm	About 420 ppm	Every 1–2 weeks
Magnesium	1,250–1,350 ppm	About 1,300–1,350 ppm	Every 2–4 weeks
Nitrate	2–20 ppm	Usually below 20 ppm, not zero	Weekly
Phosphate	0.01–0.10 ppm	Detectable and generally below 0.10 ppm	Weekly
Ammonia / nitrite	Undetectable after cycling	Zero on a reliable test	During cycling or troubleshooting

Specific gravity changes with temperature and the instrument used. A calibrated refractometer reading in ppt is the clearest way to avoid confusing units.

What I Actually Test on My Own Aquariums

I do not test every parameter every day. I monitor temperature continuously, check salinity at least weekly, and test alkalinity, nitrate, and phosphate every week because those numbers help me see whether the aquarium is changing.

I test calcium occasionally and magnesium less frequently. During a new cycle, I test ammonia and nitrite. After the cycle is established, I only return to those tests when I am troubleshooting an unexpected problem.

If you are choosing equipment, start with my saltwater aquarium test-kit guide and salinity tester guide. You do not need every checker on day one.

1. Salinity: Measure It Before You Correct Anything Else

Evaporation removes freshwater and leaves salt behind, so salinity rises unless you replace evaporated water with fresh RO/DI water. A water change can also move salinity if the new batch does not match the aquarium.

I use a refractometer with 35 ppt calibration fluid. I previously used a digital salinity tester, but after it eventually gave me incorrect readings—even after calibration—I returned to the refractometer. It is simple, dependable, and easy to verify.

Calibrate the instrument with the correct calibration fluid.
Measure freshly mixed saltwater before it enters the aquarium.
Replace evaporation with fresh RO/DI water, not saltwater.
If a result looks wrong, confirm it before changing the tank.

Related guides: RO/DI systems, salt mixes, and salinity testers.

2. Temperature: Stability Matters More Than Picking the Perfect Degree

I normally keep my aquariums around 77–78°F. The acceptable range is broader, but rapid changes are what I work hardest to prevent. A heater controller gives me an independent cutoff and an alarm if the water gets too warm or too cold.

In Palm Desert, cooling matters too. A fan can lower aquarium temperature through evaporation, but it increases top-off demand. Keep the aquarium away from direct sun and HVAC vents, use a reliable thermometer, and make sure the heater cannot touch anything that traps heat.

See the aquarium heater guide and controller guide.

3. Alkalinity: My Most Useful Reef-Tank Trend

Alkalinity is the reef parameter I watch most closely once corals begin consuming carbonate. It helps buffer pH and supplies carbonate used by calcifying organisms. I aim for about 9 dKH, but the exact number matters less than keeping it stable inside a sensible range.

If alkalinity is moving, test at the same time of day and look at several results before changing a dose. When I use kalkwasser, it adds calcium and alkalinity together and can raise pH, but it is not a universal fix. Any dosing method must be matched to measured consumption.

My preferred test is the Hanna alkalinity checker because it gives me a clear digital result. Whatever test you use, follow its timing and sample-volume instructions exactly.

4. Calcium and Magnesium: Important, but Usually Slower-Moving

Stony corals, coralline algae, clams, and other calcifying organisms use calcium and carbonate to build calcium-carbonate structures. Magnesium helps keep calcium and alkalinity available in seawater rather than precipitating too easily.

I generally target 400–450 ppm calcium and 1,250–1,350 ppm magnesium. In my tanks, alkalinity usually shows changing consumption sooner, so I test calcium less often and magnesium less often still. A new salt mix, a major dosing change, or an unexpected alkalinity problem is a good reason to check both.

Do not correct magnesium, calcium, and alkalinity simultaneously based on one surprising test. Confirm salinity first, repeat the test, and then adjust one thing at a time.

5. pH: Investigate the Cause Instead of Pouring In a Buffer

pH changes naturally during the day as photosynthesis and respiration change carbon dioxide levels. A closed home with people and pets can hold more CO₂ and push aquarium pH lower. That is common in my own home during periods when the windows stay closed.

A reading around 7.8–8.3 can be workable when it is stable, but a sudden change deserves attention. Before dosing anything, confirm the test, check alkalinity, improve gas exchange, and compare indoor and outdoor air conditions. Opening a window or improving surface agitation can sometimes explain more than a bottle of buffer.

I do not recommend blindly dosing a pH buffer. It can move alkalinity while leaving the underlying CO₂ problem unchanged.

6. Nitrate and Phosphate: Nutrients Should Be Controlled, Not Erased

Nitrate and phosphate are nutrients. Too much can contribute to algae and other problems, but driving either one to zero can also destabilize a reef aquarium. I want both to remain detectable and reasonably consistent.

For a beginner mixed reef, 2–20 ppm nitrate and 0.01–0.10 ppm phosphate are practical working ranges. My own goal is usually nitrate below 20 ppm and phosphate below about 0.10 ppm, but I respond to the trend and the animals—not one isolated decimal.

Confirm an unexpected result before changing filtration.
Remove trapped detritus and correct overfeeding before adding aggressive chemical media.
Start conservatively with GFO or another phosphate remover.
Do not make nitrate and phosphate corrections on the same day unless there is an urgent reason.

7. Ammonia and Nitrite: Cycling and Troubleshooting Tests

I test ammonia and nitrite while cycling a new aquarium. Once the biological filter is established, both should be undetectable on a reliable hobby test. I do not keep testing them every week in a stable, stocked aquarium.

If ammonia appears unexpectedly, protect the animals first. Confirm the result, check for a dead animal or interrupted filtration, increase aeration, and prepare a water change. The correct response depends on the reading, pH, temperature, and livestock, so do not treat one color-chart result as the whole diagnosis.

For this stage, the API Saltwater Master Test Kit remains my practical beginner choice because it covers ammonia and nitrite without requiring expensive single-parameter instruments.

A Testing Schedule That Beginners Can Actually Maintain

When	What to test	Why
Continuously or daily glance	Temperature; equipment alarms	Catches heater, cooling, and power problems quickly
Weekly	Salinity, alkalinity, nitrate, phosphate	Shows the trends I use most often
Every 1–2 weeks	Calcium	Confirms balanced major-element consumption
Every 2–4 weeks	Magnesium	Useful when dosing, salt mix, or precipitation changes
During cycling	Ammonia, nitrite, nitrate	Tracks the biological filter becoming established
When troubleshooting	Repeat the relevant test and verify salinity	Prevents a bad test from creating a bad correction

Watch the Water-Testing Walkthrough

This video shows the practical testing routine behind the numbers in this guide. Use it to compare technique, then follow the exact instructions supplied with your own test kit.

What to Do When a Parameter Is Out of Range

Do not dose immediately. Look at the animal behavior and the size of the change.
Repeat the test. Check the reagent, sample volume, timing, and instrument calibration.
Verify salinity. Concentrated or diluted seawater can shift several readings together.
Review your recent changes. New salt, dosing, feeding, filtration, top-off, and water changes are common clues.
Make one conservative correction. Large, overlapping changes make the cause harder to identify.
Retest after the correction has mixed fully. Record the result and watch the trend.

For a routine correction, use my step-by-step saltwater water-change guide. To choose the equipment, compare the eight testing tools I actually use.