If your corals look “fine” but growth stalls, or coralline algae stops spreading, calcium (Ca) and magnesium (Mg) are two of the first reef tank parameters to verify. The goal isn’t perfect numbers every day—it’s stable, repeatable results you can maintain without stress.
Below is a beginner-friendly guide to calcium and magnesium in reef tank care: target ranges, how to test reliably, how to interpret results, and safe correction steps that avoid cloudy “snowstorms” and big swings.
Why calcium and magnesium matter in a reef tank
What calcium does (skeleton growth, coralline algae)
Calcium is a major building block for:
- Stony coral skeleton growth (SPS and LPS)
- Coralline algae (purple/pink crust on rocks and glass)
- Many calcifying organisms (snails, clams)
In reef tanks, calcium is typically measured as ppm (mg/L).
What magnesium does (stability, prevents “calcium crashes”)
Magnesium helps keep calcium and carbonate chemistry behaving predictably. In practical terms, adequate Mg:
- Reduces the tendency for calcium carbonate to precipitate (turn into “snow”)
- Helps you maintain stable alkalinity and calcium without constant corrections
Magnesium is also measured in ppm (mg/L).
How calcium, alkalinity, and magnesium interact (simple mental model)
A simple way to think about the calcium magnesium alkalinity relationship:
- Alkalinity (dKH) = the “fuel” for calcification.
- Calcium (ppm) = the “building material.”
- Magnesium (ppm) = the “stabilizer” that helps keep Ca/Alk in solution.
If you push one hard (especially alkalinity) while Mg is low, you can trigger precipitation and see calcium and alkalinity drop together.
Target values for beginners (and what “stable” means)
Recommended target ranges
For most beginner-friendly mixed reefs:
- Calcium: 400–450 ppm
- Magnesium: 1250–1400 ppm (aim near ~1300–1350 ppm)
Safety note: Always verify salinity before reacting to Ca/Mg results. A swing from 35 ppt to 33 ppt can change readings and lead to overdosing.
When “perfect numbers” matter less than consistency
It’s better to hold:
- Calcium at 410–430 ppm steadily than to bounce between 390 and 470 ppm.
Most reef problems come from rapid changes, not being 10–20 ppm away from a target.
Signs your tank is consuming Ca/Mg (SPS, LPS, coralline)
Your tank likely has measurable consumption if you see:
- SPS tips growing / encrusting bases expanding
- LPS laying down new skeleton (heads expanding over time)
- Coralline algae spreading quickly
- You need more frequent alkalinity adjustments (often rises in demand come with Ca demand too)
Testing calcium and magnesium without frustration
Test kit options (titration vs digital) and what to expect
Common options for how to test calcium reef aquarium and magnesium:
- Titration kits (drop-by-drop color change): accurate when done carefully, slower.
- Digital colorimeters (where available): faster reads, but still sensitive to clean technique and reagent handling.
Expectation-setting: a small amount of variation is normal. Many hobby kits are most useful for trends, not single “perfect” numbers.
Best testing routine for beginners
A practical schedule that matches reef tank parameters stability needs:
- New tank (first 1–3 months):
- Ca/Mg: every 1–2 weeks
- Alk: 2–3x per week (alk tends to move first)
- Mixed reef (softies + LPS + some SPS):
- Ca: weekly
- Mg: every 2–4 weeks
- SPS-heavy:
- Ca: 1–2x per week
- Mg: weekly to biweekly (especially if corrections are frequent)
How to take a clean sample (avoiding bubbles, residue, timing)
For more consistent readings:
- Rinse the vial with tank water first (not tap water).
- Pull water from a consistent spot (same depth/area).
- Avoid bubbles in syringes (bubbles can change titration volume).
- Test at a similar time of day, especially if your pH swings are large.
Common testing errors (endpoint color, dirty syringes, lighting)
Common testing mistakes reef aquarium keepers run into:
- Reading the endpoint too early (especially on calcium kits)
- Using stained/dirty vials or syringes
- Testing under very warm or very blue lighting (color endpoints look different)
- Miscounting drops or not holding the bottle vertical
- Not wiping fingerprints off cuvettes (for digital testers)
How to log results to spot trends (not one-off readings)
Don’t just write “Ca 410.” Log:
- Ca (ppm), Mg (ppm), Alk (dKH), salinity (ppt), and what you dosed
- Notes like “new salt batch,” “added frags,” “changed dosing schedule”
Trends answer “is this drifting?” much better than a single test.
Interpreting results: “Is this number actually a problem?”
What low calcium usually means (consumption, dosing mismatch)
Low calcium often points to:
- Increased consumption (more coral growth than before)
- Two-part dosing mismatch (alk kept up, calcium not)
- Underestimated true water volume (dosing less than you think)
What low magnesium usually means (salt mix, water changes, uptake)
Low magnesium often points to:
- Salt mix that mixes low at your salinity
- Infrequent water changes (if you rely on them for replenishment)
- Gradual consumption/locking into rock and sand over time
What high calcium or magnesium can indicate
High Ca or Mg can come from:
- Overcorrection (dosing too much, too fast)
- Salinity higher than you think (e.g., 37–38 ppt)
- Inconsistent testing technique
Safety note: Very high magnesium is sometimes used intentionally for specific pest strategies online, but that approach can stress invertebrates. For beginners, focus on normal reef ranges.
Quick decision table (Hold / Retest / Correct)
| Situation | What it often means | What to do next |
|---|---|---|
| Ca slightly low (e.g., 390–399 ppm) but stable Alk | Minor deficit or test variance | Retest, then correct gently if confirmed |
| Ca low (e.g., <390 ppm) and Alk falling | Demand exceeds dosing | Correct Ca, verify Alk plan, check salinity |
| Mg slightly low (e.g., 1200–1249 ppm) | Salt mix/water change gap | Retest, then dose Mg in steps |
| Mg low (<1200 ppm) with frequent Ca/Alk instability | Low stabilizer increases precipitation risk | Correct Mg first in measured steps |
| Ca high (>470 ppm) or Mg high (>1500 ppm) | Overdosing or salinity error | Pause dosing, re-check salinity, retest |
Correcting calcium safely (step-by-step)
Choose your approach
For dosing calcium reef tank corrections, pick the least stressful method that fits the size of the correction.
- Water changes (small corrections)
- Best when you’re only a little low and your salt mix matches your targets at 35 ppt.
- Two-part dosing (ongoing stability)
- Great for mixed reefs and SPS as demand grows.
- Designed to maintain both alkalinity and calcium over time.
- Calcium chloride “one-time” correction
- Useful to bring calcium back into range, then maintain with two-part or water changes.
Safe correction pace (avoid big swings)
Practical beginner guidance: raise calcium gradually rather than in one big jump.
- Aim to increase calcium in measured steps, then retest.
- If corals look irritated (retracted polyps, tissue looks tight), slow down and confirm salinity and alkalinity.
How to calculate a dose (what you need to know)
To dose safely, you need:
- True water volume (display + sump minus rock/sand displacement)
- The product’s concentration (follow the label)
- Your target increase (example: from 380 → 420 ppm)
If you’re unsure of true volume, assume you have less water than the tank’s advertised size. Overestimating volume is a common cause of overdosing.
Where and how to add (high flow, spaced dosing, avoid precipitation)
To reduce clouding and precipitation:
- Add calcium solution to a high-flow area (return section or strong powerhead zone).
- Dose slowly, not all at once.
- If you also dose alkalinity, separate the additions (different times/areas). Mixing Ca and Alk concentrates can create instant precipitation.
Correcting magnesium safely (step-by-step)
Magnesium chloride vs magnesium sulfate (and why mixes are common)
For dosing magnesium reef tank, you’ll typically see:
- Magnesium chloride
- Magnesium sulfate
- Or a blended product (common because it helps avoid shifting the chloride:sulfate balance too far over time)
If you’re using raw chemicals, follow a well-established reef chemistry recipe and measure carefully. Beginner-friendly commercial mixes are often simpler and safer.
Safe correction pace
Increase magnesium in measured steps, with retesting between doses.
- Make one adjustment, let it mix fully, then retest before repeating.
- This avoids overshooting and helps you confirm your test technique.
Dosing tips to avoid cloudy water or irritated corals
- Dose into high flow.
- Avoid dumping powder directly into the tank (pre-dissolve if required).
- If water turns cloudy, stop dosing and check:
- pH (high pH increases precipitation risk)
- alkalinity (very high dKH can combine with calcium to precipitate)
- salinity (ppt)
Preventing “snowstorms” and parameter swings
What causes precipitation (high pH, overdosing, adding too fast)
Cloudy water (“snow”) is usually calcium carbonate precipitation, commonly triggered by:
- Adding calcium and alkalinity too close together
- High pH events (e.g., heavy kalkwasser use)
- Dosing too quickly into low-flow areas
- Running alkalinity high while Mg is low
The #1 stability rule: don’t chase daily noise
Don’t change dosing based on one test unless it’s clearly out of range.
- Confirm with a retest
- Look at the trend over a week
- Make small changes, then re-evaluate
Build a simple maintenance plan
A stable beginner plan looks like:
- Weekly testing cadence: Alk weekly (or more), Ca weekly, Mg monthly (or more if adjusting)
- Dosing schedule: same time daily, consistent amounts
- Water change strategy: consistent schedule (example: 10–15% weekly or biweekly), mixed to 35 ppt, heated and circulated
Beginner troubleshooting guide
Calcium won’t rise even after dosing
Check these, in order:
- Salinity (ppt): if it’s low, your Ca may appear low and dosing may be inefficient
- Alkalinity (dKH): very high dKH can increase precipitation risk
- Dosing method: are you dosing into high flow? spacing Ca and Alk?
- Test kit technique: confirm endpoint and reagent condition
Magnesium keeps drifting down
Common causes:
- Salt mix that tests lower Mg at your chosen salinity
- Large water changes with a lower-Mg mix
- Under-dosing due to underestimated water volume
Fix:
- Test freshly mixed saltwater at 35 ppt for Mg (ppm)
- Adjust your maintenance dosing or pick a salt mix that matches your targets
Alkalinity is stable but calcium isn’t (and vice versa)
This often happens when:
- Two-part dosing is not balanced to your tank’s real consumption
- You’re doing frequent water changes (which can add Ca/Mg) but dosing Alk aggressively
- A test error is masking the true relationship
Tip: Adjust dosing based on alkalinity first for day-to-day stability, but don’t ignore calcium—bring it back into range, then maintain.
When to pause dosing and re-check salinity
Pause and verify salinity if:
- Ca and Mg both seem “off” compared to your salt mix
- Numbers swing wildly despite consistent dosing
- You recently changed your ATO, refractometer calibration, or salt brand
Use a calibrated instrument and aim for 35 ppt (1.0264 SG at 25°C / 77°F).
Quick reference: beginner targets and actions
One-page summary table
| Parameter | Beginner target | Typical testing frequency | “Safe change” mindset | Retest interval |
|---|---|---|---|---|
| Calcium (ppm) | 400–450 | Weekly (mixed reef) | Gradual increases, avoid big jumps | After each correction step |
| Magnesium (ppm) | 1250–1400 (aim 1300–1350) | Every 2–4 weeks | Stepwise adjustments | After each correction step |
| Alkalinity (dKH) | (match your chosen system; keep stable) | 2–3x/week to weekly | Avoid rapid swings | 24–48 hrs after dose changes |
| Salinity (ppt) | 35 ppt | Weekly (and after changes) | Stability matters most | Immediately if results look “weird” |
What to log in an app (Ca/Mg/Alk/salinity, dosing amounts, notes)
To make your testing actually useful, log:
- Calcium (ppm), Magnesium (ppm), Alkalinity (dKH), Salinity (ppt)
- Dosing amounts (mL or grams), product name, and time dosed
- Notes: water changes, new salt bucket, livestock additions, equipment changes
This makes it much easier to spot whether you have a real drift or just test noise.
FAQ
What is the ideal calcium level in a reef tank?
For most reef aquariums, a solid beginner range is 400–450 ppm calcium. Choose a target in that range and prioritize keeping it stable.
What is the ideal magnesium level in a reef aquarium?
A common target range is 1250–1400 ppm, with many beginners having the easiest stability around 1300–1350 ppm.
How do I raise calcium in a reef tank safely?
Use a calcium chloride product for a one-time correction or two-part dosing for ongoing maintenance. Dose in high flow, raise it gradually, and retest between steps.
How do I raise magnesium in a reef tank safely?
Use a reef magnesium supplement (often a chloride/sulfate blend). Increase Mg in measured steps, dosing into high flow, and retest before adding more.
Should I correct calcium or alkalinity first?
If magnesium is low (e.g., near <1200 ppm), correct magnesium first to reduce instability. For day-to-day control, many reefers adjust alkalinity (dKH) first, then bring calcium back into range and maintain both.
How often should I test calcium and magnesium in a reef tank?
For a mixed reef, test calcium weekly and magnesium every 2–4 weeks. Test more often during growth spurts, after major changes, or while dialing in dosing.
Call to action: make stability easier
If you want fewer surprises, the biggest upgrade is simple tracking. Log Ca (ppm), Mg (ppm), Alk (dKH), salinity (ppt), and every dose so you can see trends before corals get stressed.
Use Reef Buddy to track your calcium and magnesium tests, dosing amounts, and maintenance reminders in one place. And if you’re unsure what to do next, Shrimpy can help you interpret your numbers and choose a low-stress correction plan.