Every parameter that matters in a reef tank, target ranges, how to test
accurately, how often to test at each stage, what your results are telling
you, and what to actually do when something is out of range.
Not what you think is happening, not what the tank looks like, what the
chemistry is actually doing. A tank can look clear, calm, and healthy while
alkalinity has been slowly dropping for three weeks, or while phosphate has
climbed high enough to inhibit coral calcification. The tank doesn’t tell you.
The test kit does.Most beginner water testing advice focuses on what to test and ignores the
harder questions: how to test accurately enough that the results are actually
meaningful, how to read trends rather than just single numbers, and what to
do with a result that’s out of range. This guide covers all of it.
Parameter Reference, Target Ranges at a Glance
| Parameter | Target Range | Test Frequency | Priority |
|---|---|---|---|
| Salinity | 1.025–1.026 SG (35 ppt) | Weekly | Critical |
| Temperature | 77–79°F (25–26°C) | Daily | Critical |
| Ammonia (NH₃) | 0 ppm | During cycle; spot-check after additions | Critical |
| Nitrite (NO₂) | 0 ppm | During cycle; spot-check if ammonia rises | Critical |
| Nitrate (NO₃) | <10 ppm (reef); <20 ppm (FOWLR) | Weekly | High |
| Alkalinity (dKH) | 8–10 dKH | Weekly | High (critical with corals) |
| Calcium (Ca) | 400–450 ppm | Weekly once corals established | High (with stony corals) |
| Magnesium (Mg) | 1250–1350 ppm | Monthly (weekly if instability) | Moderate |
| Phosphate (PO₄) | <0.05 ppm (reef); <0.1 ppm (FOWLR) | Weekly | High |
| pH | 8.1–8.3 | Weekly; monitor trend not single values | Moderate |
Testing Accurately, The Details That Matter
A test result is only useful if it’s accurate. Most beginner testing errors
don’t come from bad test kits, they come from technique. Here’s what actually
affects result accuracy:
Sample Collection
- Don’t test surface water. The surface film concentrates organics, oils, and evaporation residue that skew results. Collect your sample from mid-water depth, away from the return pump output and wavemaker, areas with extreme turbulence give less representative samples.
- Collect after the lights have been on for at least 2 hours. pH in particular fluctuates significantly between lights-on and lights-off as corals and algae consume and produce CO₂. Testing pH consistently at the same time of day gives comparable results. Testing at random times of day does not.
- Use a clean, dry collection vessel. Residual water from a previous test can dilute or contaminate the sample. Rinse the tube with tank water before collecting the test sample and discard the rinse water.
Liquid Test Kit Technique
- Count reagent drops carefully. A difference of one or two drops can shift results by a meaningful amount, particularly for alkalinity titration tests. Hold the dropper bottle vertically, let each drop fall fully before counting the next.
- Shake reagent bottles before use. Some reagents settle between uses, the API Nitrate test Bottle #2 is the most widely known example and requires 30 full seconds of vigorous shaking for accurate results. Skipping this step is the most common cause of inaccurate nitrate readings with this kit.
- Read color results in consistent lighting. The color-matching in liquid test kits is affected by ambient light. Read color results in natural daylight or under a consistent artificial light source, not under blue reef lighting, which distorts color perception entirely.
- Read results at the correct time. Most liquid kits specify a wait time (typically 5 minutes) before reading color results. Reading early produces lighter, lower readings. Follow the timing in the kit instructions exactly.
Refractometer Calibration
- Calibrate your refractometer with RODI water (should read 1.000 or 0 ppt) before each testing session or at minimum once per week.
- Allow the refractometer to reach ambient room temperature before calibrating and testing, cold refractometers read low.
- Swing-arm hydrometers are not sufficiently accurate for reef keeping. If you’re still using one, replace it with a refractometer.
The Golden Rule of Water Testing
Never make a significant tank change based on a single test result. If a
parameter reads unexpectedly high or low, retest before acting, ideally
with a second kit or method. A false positive on alkalinity that triggers
an aggressive dosing response can cause more damage than the original
reading would have. Verify first, correct second.
Every Parameter Explained
Salinity
Salinity measures the concentration of dissolved salt in the water. In a reef
tank, this is maintained at natural seawater levels, 1.025–1.026 specific
gravity, or approximately 35 ppt. Fish and corals are adapted to this range
and experience osmotic stress when it deviates significantly in either direction.
How to Test
Use a temperature-compensating refractometer. Place 2–3 drops of tank water
on the prism, close the daylight plate, and read the scale through the eyepiece.
The boundary line between light and dark indicates specific gravity on one scale
and ppt on the other. Calibrate with RODI water before testing.
Target: 1.025–1.026 SG
- Below 1.024: Hyposalinity, stresses corals and invertebrates. Raise slowly by increasing salt concentration in top-off water over several days.
- Above 1.027: Hypersalinity, usually from insufficient top-off. Correct by increasing RODI top-off volume over 3–5 days. Never correct salinity quickly in either direction, osmotic shock is more damaging than the elevated salinity itself.
How Often to Test
Weekly, and any time evaporation has been unusually high or top-off has been missed for several days. An ATO eliminates most salinity variance automatically. See: Best Auto Top Off Systems
Temperature
Temperature affects every biological process in a reef tank, bacterial activity,
coral metabolism, oxygen saturation, and disease susceptibility. The target range
of 77–79°F (25–26°C) supports the widest range of common reef organisms without
pushing to the thermal stress boundary that most corals approach above 82°F.
How to Test
Use an independent digital thermometer, not the heater dial, which is a target
setting, not an accurate measurement. Verify the thermometer itself against a
second reference at setup. A $10 digital thermometer with a probe is reliable.
Avoid stick-on strip thermometers, they’re affected by ambient temperature and
read inaccurately.
Target: 77–79°F
- Swing greater than 2°F in 24 hours: Thermal stress, investigate heater calibration, room temperature changes, and lighting heat output.
- Consistently above 82°F: Coral bleaching risk, requires intervention (chiller, fans, reduced lighting).
- Below 75°F: Fish and coral immune suppression, verify heater is functioning and sized correctly.
How Often to Test
Daily visual check of the thermometer reading. Log weekly. See: Reef Tank Temperature and Stability
Ammonia (NH₃/NH₄⁺)
Ammonia is the primary toxic waste product produced by fish respiration,
uneaten food decomposition, and the breakdown of organic matter. In a fully
cycled, established reef tank it should always read 0 ppm. Any detectable
ammonia in an established tank indicates a problem, a large die-off, a crash
of biological filtration, or a sudden overload of the nitrogen cycle.
How to Test
Use a liquid test kit (API Saltwater Master or equivalent). Add the specified
number of drops to the water sample, shake, and wait the prescribed time before
reading. The API kit reads both ammonia (NH₃) and ammonium (NH₄⁺) together, in a reef tank, any reading above 0 is a concern regardless of which form is present.
Target: 0 ppm
- 0.25 ppm or above: Immediate concern in an established tank. Do a 20–25% water change, identify and remove any dead livestock, and retest in 24 hours.
- Persistent ammonia in an established tank: Biological filtration may have been disrupted, check if any medication, copper treatment, or cleaning product has entered the tank.
How Often to Test
Every 2–3 days during cycling. Monthly spot-check in an established tank. Immediately after any suspected livestock death or disease treatment.
Nitrite (NO₂⁻)
Nitrite is the intermediate product of the nitrogen cycle, produced when
Nitrosomonas bacteria process ammonia. Like ammonia, it should read 0 ppm in a
fully cycled, established tank. Nitrite interferes with fish blood’s ability
to carry oxygen, causing suffocation even in well-oxygenated water.
How to Test
Liquid test kit (included in the API Saltwater Master kit). Same technique
as ammonia, drops, shake, wait, read color against chart in consistent lighting.
Target: 0 ppm
- Any reading above 0 in an established tank: Same response as ammonia, water change, identify cause, retest.
- Nitrite rising during cycling: Expected, it means the first stage of the cycle is working. It will peak and then fall as Nitrospira bacteria colonize.
How Often to Test
Every 2–3 days during cycling. Monthly spot-check in an established tank. Always test alongside ammonia when either is suspected.
Nitrate (NO₃⁻)
Nitrate is the end product of the nitrogen cycle, far less toxic than ammonia
or nitrite but not harmless. At elevated levels it contributes to algae growth,
reduces coral coloration and growth rate, and over time degrades the health of
the overall system. In a reef tank, keeping nitrate below 10 ppm is the goal.
A FOWLR (fish-only with live rock) tank can tolerate up to 20–30 ppm without
immediate livestock effects, but lower is always better.
How to Test
Liquid test kit, the API nitrate test requires vigorous shaking of Bottle #2
for 30 full seconds before use. This is not a suggestion, it’s a calibration
step. Skipping it produces readings that are consistently low and unreliable.
Salifert nitrate test is an alternative with a simpler protocol.
Target: Under 10 ppm
- 10–20 ppm: Acceptable for fish-only tanks; concerning for reef. Increase water change frequency.
- Above 20 ppm: Actively degrading coral health. Identify and address the source, overfeeding, old filter floss, insufficient skimming, or excess bioload. Do not attempt to correct with large emergency water changes, make small increases in change frequency instead.
- 0 ppm consistently: Can actually be too low in a tank with soft corals and zoanthids that benefit from slightly elevated nutrients (2–5 ppm). Completely undetectable nitrate in a fed, stocked tank sometimes indicates a test kit problem or very aggressive nutrient export.
How Often to Test
Weekly. Track the trend, a nitrate that’s stable at 8 ppm week over week is very different from one that’s climbing from 5 to 8 to 11 ppm over three weeks.
Alkalinity (dKH)
Alkalinity, measured in dKH (degrees of carbonate hardness), is the most
important parameter to track consistently in a reef tank with corals. It measures
the buffering capacity of the water and is the primary building block of coral
skeletons. Stony corals (LPS and SPS) consume alkalinity continuously as they
grow, in a well-stocked reef, consumption can be significant enough to drop
alkalinity measurably within a week.
Stability matters more than the exact target number. A tank that holds 8.5 dKH
consistently will produce healthier corals than a tank that bounces between
7.5 and 10 dKH, even though 10 dKH is technically within the ideal range.
Rapid alkalinity swings cause coral tissue recession, bleaching, and in acute
cases, a spike or crash of more than 2 dKH in 24 hours, rapid coral death.
How to Test
Two options with meaningfully different accuracy levels:
- Salifert Alkalinity Test Kit, a titration method where you
add reagent drop by drop until a color change endpoint is reached. Each drop
represents approximately 0.5 dKH. Reliable and well-regarded as liquid kits go.
Best used consistently by the same person, technique variation between testers
introduces variance. - Hanna HI755 Alkalinity Checker, a digital colorimeter that
produces a numerical readout without color matching. Significantly more accurate
and consistent than liquid kits, particularly for detecting small changes
(0.1–0.2 dKH) between weekly tests. The best single equipment upgrade for
a beginner’s testing routine.
See: Best Reef Tank Test Kits for Beginners
Target: 8–10 dKH, stable
- Dropping more than 0.5 dKH week over week: Coral consumption is exceeding natural replenishment from water changes. Time to begin dosing (two-part or kalkwasser). See the dosing guide for how to start safely.
- Above 11 dKH: Overdosing or excessive salt mix alkalinity. Reduce slowly, do not crash it down quickly.
- Below 7 dKH: Coral tissue recession risk. Raise by no more than 0.5 dKH per day using a diluted alkalinity supplement.
How Often to Test
Weekly minimum once any coral is in the tank. Twice weekly when first establishing dosing or when making changes to the coral collection.
Calcium (Ca)
Calcium is the other primary building block of coral skeletons, consumed
alongside alkalinity in a roughly consistent ratio. In a tank with only soft
corals, calcium consumption is minimal. In a tank with LPS or SPS corals,
calcium and alkalinity must both be monitored and maintained together, a
tank that’s dosing alkalinity correctly but neglecting calcium will eventually
see coral growth rate slow as available calcium is depleted.
The alkalinity-to-calcium relationship is maintained naturally in natural
seawater, quality reef salt mixes are formulated to approximate this balance.
Regular water changes alone maintain both parameters in lightly stocked tanks.
As coral demand grows, supplementation becomes necessary.
How to Test
Salifert Calcium Test or Hanna HI758 Calcium Checker. The Salifert calcium
test uses a titration endpoint method and requires careful technique, the color
change from pink to purple is distinct, but the endpoint can be missed if drops
are added too quickly. Add drops slowly near the endpoint.
Target: 400–450 ppm
- Below 380 ppm: Coral calcification slows. Raise gradually using a calcium supplement, never more than 20 ppm per day without confirming alkalinity is in balance.
- Above 480 ppm: Calcium and alkalinity chemistry becomes unstable, precipitation risk (calcium carbonate spontaneously precipitating out of solution, crashing both parameters simultaneously). Reduce through water changes only.
- Calcium and alkalinity moving inversely: If calcium rises when you dose alkalinity, or vice versa, the two parameters are out of ionic balance. A magnesium test is the next step, magnesium deficiency disrupts calcium-alkalinity chemistry.
How Often to Test
Weekly once LPS or SPS corals are established. Monthly for soft coral-only tanks.
Magnesium (Mg)
Magnesium is the third element of the reef chemistry triad, less immediately
visible in its effects than alkalinity or calcium, but critical for maintaining
stable chemistry between the two. Magnesium inhibits the spontaneous precipitation
of calcium carbonate, keeping calcium and alkalinity in solution and available
to corals. A magnesium-deficient tank is one where alkalinity and calcium are
persistently difficult to maintain despite regular dosing.
How to Test
Salifert Magnesium Test. The protocol involves two reagent additions and an
endpoint color change, the most complex of the three triad tests, requiring
careful technique. Add reagent slowly at the endpoint and wait 5 seconds between
drops. A Hanna HI783 Magnesium Checker is available for digital accuracy.
Target: 1250–1350 ppm
- Below 1200 ppm: Calcium and alkalinity will be difficult to stabilize regardless of dosing. Raise magnesium first before addressing the other parameters.
- Above 1400 ppm: Rarely a problem from overdosing but can occur from some salt mixes. Allow to decline naturally through water changes.
How Often to Test
Monthly in a stable tank. Test immediately if alkalinity or calcium are unexpectedly difficult to stabilize.
Phosphate (PO₄)
Phosphate is a nutrient that drives algae growth and, at elevated levels,
inhibits coral calcification, the same biological process that phosphate
promotes in plants (it’s a primary plant nutrient) actively works against
coral skeleton formation. Keeping phosphate under 0.05 ppm in a reef tank
with corals is the target, below 0.03 ppm for SPS-dominated systems.
The challenge with phosphate testing is accuracy at reef-relevant concentrations.
Most liquid test kits, including Salifert’s phosphate test, are designed to
measure down to 0.1 ppm, which is already too high for a serious reef tank.
A result of “undetectable” on a standard liquid kit could mean anywhere from
0 to 0.09 ppm, a range that matters enormously for SPS coral health.
How to Test
For accurate reef-level phosphate measurement: Hanna HI736
Ultra Low Range (ULR) Phosphate Checker. Reads down to 0.01 ppm using a
digital colorimeter method, the only practical way to accurately track
phosphate at the concentrations a serious reef requires. This is the most
impactful single equipment upgrade for anyone keeping SPS corals.
For general monitoring in a soft coral tank: Salifert
Phosphate Test is adequate for detecting elevated levels (above 0.1 ppm)
that indicate a nutrient problem worth investigating.
Target: Under 0.05 ppm
- 0.05–0.1 ppm: Acceptable for soft coral and LPS tanks; too high for SPS. Address through increased water change frequency, reduced feeding, and filter floss replacement.
- Above 0.1 ppm: Nuisance algae territory. Identify and address the source. GFO media can export phosphate quickly but should be introduced gradually, a sudden drop from 0.1 to 0.01 ppm shocks corals that have adapted to the elevated level.
- Undetectable on Hanna ULR (<0.01 ppm): Too low for most corals, some nutrient is required. Slightly elevated feeding or reduced water change frequency typically corrects this.
How Often to Test
Weekly in an established reef. Any time nuisance algae appears or coral color loss is observed.
pH
pH measures the acidity or alkalinity of the water on a logarithmic scale from
0 to 14. Reef tanks target 8.1–8.3. Unlike the parameters above, pH fluctuates
naturally and predictably throughout the day, it rises as photosynthesis
consumes CO₂ during the photoperiod and falls overnight as respiration produces
CO₂ without the offsetting effect of photosynthesis. A daily swing of 0.1–0.2 pH
units is completely normal. A swing of more than 0.3 units suggests a gas
exchange problem.
pH is the most commonly over-tested and over-corrected parameter in beginner
reef tanks. A single low pH reading is almost never an emergency, it’s usually
a snapshot of the overnight low point before photosynthesis has had time to
pull CO₂ back down. Chase the trend, not the single reading.
How to Test
Liquid test kit (API High Range pH included in the Saltwater Master kit) or
a digital pH probe/controller. For trend monitoring, a digital probe that logs
continuously is far more informative than a liquid test taken once per week, it shows you the full daily swing rather than a snapshot. Milwaukee MW102 and
Apex pH probes are common beginner options.
Test pH at the same time each day for comparable results, ideally 2–3 hours
after lights on (when CO₂ has had time to decrease) for a representative reading.
Target: 8.1–8.3, stable
- Consistently below 7.9: Poor gas exchange, CO₂ is accumulating. Increase surface agitation, add a skimmer if not present, or route the skimmer air intake outside if the room CO₂ is elevated.
- Daily swing greater than 0.3 units: Excessive swing suggests imbalanced CO₂, usually a combination of low surface agitation and high biological activity. Address flow before dosing pH buffers.
- Consistently above 8.4: Rare without active pH manipulation. Can occur with heavy kalkwasser dosing, reduce dose rate.
How Often to Test
Weekly at the same time of day. Consider a continuous pH probe if you want to monitor the full daily swing, the trend is more informative than any single test.
Testing Schedule by Tank Stage
The right testing frequency depends on where the tank is in its lifecycle.
Too little testing in an early-stage tank misses critical cycle data. Too
much testing in a stable tank produces anxiety without actionable information.
| Stage | What to Test | Frequency |
|---|---|---|
| Cycling (weeks 1–6) | Ammonia, nitrite, nitrate | Every 2–3 days |
| Post-cycle, fish only (months 1–3) | Ammonia, nitrate, salinity, temperature | Weekly |
| First corals added | Alkalinity, nitrate, phosphate, salinity, temperature | Weekly |
| Established reef with LPS/SPS | Alkalinity, calcium, nitrate, phosphate, salinity, temperature, pH; magnesium monthly | Weekly (alk, Ca twice weekly when dosing) |
| After any significant change | Full parameter panel | Within 24 hours of change; again 48 hours later |
“After any significant change” covers: adding new fish or coral, medicating
the tank, adjusting dosing, performing an unusually large water change, or
discovering a piece of dead livestock. Any event that changes the tank’s
biological or chemical balance warrants a full parameter check within 24 hours.
Reading Trends, Not Just Numbers
The most important shift in thinking about water testing is moving from
“what does the tank read today” to “which direction are things moving.”
A single alkalinity reading of 8.2 dKH tells you the tank is in range.
Four consecutive weekly alkalinity readings of 8.8, 8.5, 8.2, and 7.9 dKH
tell you the tank is losing 0.3 dKH per week and will be out of range
in three weeks unless something changes.
The trend is the diagnosis. The single number is just a data point.
This is why logging every result matters, not to have a record of what the
tank read last Tuesday, but to have the data to identify trends before they
become problems. A simple weekly log with a column for each parameter makes
trends visible at a glance. A notebook of disconnected readings doesn’t.
What Trends Actually Look Like
| Trend Pattern | What It Means | Action |
|---|---|---|
| Alkalinity stable at 8.5 dKH for 8 weeks | Water changes are maintaining the parameter naturally | No action needed, continue routine |
| Alkalinity dropping 0.3 dKH/week for 4 weeks | Coral consumption outpacing water changes | Begin two-part dosing |
| Nitrate stable at 5 ppm for 6 weeks, then rises to 12 ppm | Bioload or feeding increased; nutrient export falling behind | Identify cause (new fish? Less frequent water changes?); address source |
| Phosphate stable at 0.03 ppm, drops to undetectable after adding GFO | GFO is working but may be too aggressive | Reduce GFO quantity; gradual phosphate drop is safer than sudden |
| Salinity reading 1.026 for weeks, then 1.028 two consecutive tests | Top-off frequency has decreased; evaporation outpacing compensation | Increase top-off volume; investigate ATO if not already in use |
When a Parameter Is Out of Range, First Steps
Before making any correction to an out-of-range parameter:
- Verify the result. Retest with the same kit, or test with a second kit or method. One unexpected reading is worth confirming before acting on it.
- Check the trend. Is this the first time this parameter has been out of range, or is it the continuation of a drift that’s been building? The trend determines how urgently correction is needed.
- Look at livestock behavior. Are corals open and extended? Are fish behaving normally? If livestock looks healthy, a mildly out-of-range reading is less urgent than the same reading combined with visible stress.
- Identify the cause, not just the symptom. Correcting a high nitrate reading without understanding why it rose will produce a tank that needs correcting again in three weeks.
- Correct gradually. Most reef parameters should be moved at a rate that doesn’t outpace the livestock’s ability to adapt. The exception is acute toxicity, high ammonia or nitrite requires immediate large water changes.
Build the Testing Habit and Keep It
Pick a day of the week, set up the same testing station every time, and run
through the full panel in the same order. After four weeks it takes fifteen
minutes. After four months the numbers will start telling a story, and that
story is the best early warning system your tank has.
