Alkalinity (KH) is one of the fastest ways a reef tank can go from “looks great” to “why are my corals angry?” The good news: most KH problems are fixable if you test accurately, change slowly, and aim for stability instead of “perfect numbers.”
What Alkalinity (KH) Is and Why It Matters
Alkalinity is your tank’s buffering capacity—its ability to resist pH swings. In reef tanks, alkalinity is also a key ingredient corals and coralline algae use to build skeletons.
KH vs dKH vs meq/L (quick definitions)
- KH: General term hobbyists use for alkalinity.
- dKH (degrees of carbonate hardness): Most common reef unit.
- meq/L (milliequivalents per liter): Another common unit in chemistry and some test kits.
Helpful conversion:
- 1 meq/L ≈ 2.8 dKH
- 1 dKH ≈ 0.36 meq/L
How alkalinity supports pH stability and coral growth
- Buffers acids produced by biological processes, helping keep pH steadier.
- Provides carbonate/bicarbonate corals use for calcification (along with calcium and magnesium).
Why “stable” is usually better than “ideal”
A reef that holds 8.0 dKH every day usually performs better than one bouncing between 7–10 dKH, even if 10 dKH is “acceptable.” Many coral stress events come from swings, not the absolute number.
Ideal Alkalinity Range for Reef Tanks (and Why It Varies)
There isn’t one perfect KH for every reef tank. Your best target is a stable value that matches your system, salt mix, and nutrient level.
Typical targets (mixed reef, softies, SPS)
Common beginner-friendly targets:
- Soft coral / LPS-heavy: 7.5–9.0 dKH
- Mixed reef: 8.0–9.0 dKH
- SPS-dominant: 8.0–9.5 dKH (some successful tanks run 7–8 dKH, especially with lower nutrients)
General safety note: for most reefs, 7–11 dKH is workable, but rapid changes are risky.
Match your salt mix and husbandry style
- If your salt mix mixes to 11 dKH but you run the tank at 8 dKH, large water changes can cause upward swings.
- If you prefer higher alkalinity (9–10 dKH), it often pairs better with non-ultra-low nutrients (nitrate and phosphate not bottomed out).
When a “high” or “low” KH is actually a problem
- Problematic high KH is often high plus one or more of:
- Very low nutrients (near-zero nitrate/phosphate)
- Large recent dosing corrections
- Coral “burnt tips” (especially SPS)
- Problematic low KH is often low plus:
- Unstable pH (big day/night pH swings)
- Rapid daily decline from unbalanced dosing or increased coral demand
Test First: How to Measure KH Accurately
Before adjusting alkalinity, confirm the number. Many “KH emergencies” are testing errors.
Best practices for hobby test kits
- Use clean, dry vials (rinse with RO/DI, then tank water).
- Follow the kit’s timing and swirling steps.
- Read results under good lighting.
- Use a consistent method: same vial, same sample volume, same technique.
When to retest (confirming unexpected results)
Retest if:
- The result is far from your usual (example: 8.5 dKH yesterday, 12 dKH today).
- You changed anything recently (salt brand, doser settings, new supplement).
- Livestock looks normal but the value looks “wild.”
If possible, cross-check with:
- A fresh reagent
- A different brand kit
- A trusted local store test
Tracking trends (daily use, weekly stability)
- If you dose alkalinity daily, test daily for 3–7 days to find real consumption.
- Once stable, many beginners test 2–3x per week (or weekly in very stable systems).
Common testing errors (dirty vials, expired reagents, bubbles)
- Dirty vial residue can skew color endpoints.
- Expired reagents can read high or low.
- Microbubbles in syringes can change the true volume.
- Inconsistent sample size (even a small mismatch matters).
Before You Change Anything: Diagnose the Cause
Fixing the cause prevents repeat swings.
Natural alkalinity consumption (corals, coralline algae)
- As corals grow, KH demand rises.
- Coralline algae growth can noticeably increase KH use.
Dosing mismatches (alkalinity vs calcium vs magnesium)
- Alkalinity and calcium are linked. If you dose one heavily without the other, you can create instability or precipitation.
- Typical reef targets (as a reference):
- Calcium: ~400–450 mg/L
- Magnesium: ~1250–1400 mg/L
- Salinity: 35 ppt (1.026 SG)
Salt mix, water changes, and top-off water
- Water changes can raise or lower KH depending on the new saltwater’s alkalinity.
- Top-off water should be RO/DI (no salt). Using anything else can introduce alkalinity or contaminants unpredictably.
pH-driven choices: when soda ash makes sense
- If your pH is consistently low (often due to high indoor CO₂) but alkalinity is fine, raising KH is not the primary fix.
- Soda ash can temporarily boost pH, but it also raises KH—so it’s only appropriate if you actually need more alkalinity.
How to Raise Alkalinity (KH) Safely
Safe adjustment limits (avoid rapid swings)
Beginner-safe guideline:
- Raise alkalinity by no more than ~0.5–1.0 dKH per day.
- If corals are stressed, lean toward the slower end.
Safety note: large single corrections are a common cause of coral recession or “burnt tips,” especially in SPS.
Options to increase KH
- Baking soda (sodium bicarbonate)
- Gentler on pH (smaller pH rise).
- Good “default” choice for many beginners.
- Soda ash (sodium carbonate)
- Stronger pH boost.
- Can increase precipitation risk if overdosed or added too fast.
- Two-part alkalinity systems
- Designed to pair alkalinity with calcium dosing.
- Great for consistent daily demand.
- Kalkwasser (calcium hydroxide)
- Raises calcium and alkalinity together and tends to raise pH.
- Limited by evaporation rate and can be risky if added too quickly (pH spike).
Step-by-step dosing workflow (beginner process)
- Measure current KH (dKH).
- Choose a target KH that you can hold steady (often 8–9 dKH).
- Calculate the dose using your product’s directions or a reliable reef dosing calculator.
- Add slowly in a high-flow area (sump or strong display flow).
- Avoid dripping onto corals or directly onto rock where it can precipitate.
- Retest after the water mixes (often 30–60 minutes for a spot check; longer for full system equilibrium depending on setup).
- Adjust gradually over days, not minutes.
When to split doses (morning/evening)
Split dosing helps when:
- Your tank consumes a lot each day (common in SPS systems).
- You see daily KH dips and rebounds.
- You want a smoother pH curve.
A simple approach: dose half in the morning and half in the evening (or use a dosing pump for multiple small doses).
Signs you raised KH too fast
Watch for:
- SPS: pale tips, “burnt tips,” reduced polyp extension
- LPS: tissue recession, failure to inflate normally
- General: sudden precipitation (snowy water, crusty deposits on heaters/pumps)
If this happens: stop further increases, confirm the test, and stabilize.
How to Lower Alkalinity (KH) Safely
The safest method: let it fall naturally (stop/reduce dosing)
If KH is high, the safest correction is usually:
- Pause alkalinity dosing (or reduce it)
- Allow coral and coralline consumption to bring KH down gradually
This avoids chemical shock.
Use water changes strategically
Water changes can help, but only when planned.
Matching new saltwater KH to your goal
- Test your freshly mixed saltwater KH (at 35 ppt) before using it.
- If your tank is 11 dKH and you want 8.5 dKH, using a salt mix that mixes to 7 dKH can cause a fast drop—sometimes too fast.
Avoid “overshooting” with low-KH mixes
- Aim for a new-water KH close to your target, not drastically below it.
- Consider smaller, more frequent changes instead of one big change.
When (and when not) to use acids/buffers
- Acid-based “quick fixes” can cause rapid pH swings and are easy to overdo.
- For beginners, it’s best to avoid acids for lowering alkalinity unless you have a strong reason and experience.
What to do if KH is extremely high
If alkalinity is extremely high (example: >12–14 dKH):
- Stop alkalinity additives immediately.
- Confirm the reading with a retest.
- Increase monitoring (KH daily, observe corals).
- Use controlled water changes only if needed, aiming for gradual reduction.
- Keep calcium and magnesium in normal ranges to reduce precipitation risk.
Preventing Future KH Swings
Build a consistent dosing routine (manual or doser)
- Determine daily consumption: test at the same time for several days.
- Dose to replace what the tank uses each day.
- Consider a dosing pump for stability if your tank demand is consistent.
Balance alkalinity with calcium and magnesium
A balanced system is easier to keep stable:
- If alkalinity is drifting, check calcium (mg/L) and magnesium (mg/L) too.
- Avoid large corrections of multiple parameters at once.
Keep salinity stable (evaporation and ATO)
- Salinity swings change measured concentrations and can confuse your dosing plan.
- Use an ATO (auto top-off) with RO/DI water to keep salinity near 35 ppt.
Logging KH in Reef Buddy (trendlines, alerts, notes)
Tracking makes KH “boring,” which is what you want:
- Log dKH, dose amounts, and water changes.
- Add notes like “new salt brand” or “increased coral load.”
- In Reef Buddy, Shrimpy can help you spot trends, catch early swings, and build a simple routine you can follow.
Quick Reference: Common KH Scenarios and Fixes
| Scenario | What you’ll notice | Likely cause | Practical fix |
|---|---|---|---|
| KH drops daily in a new coral tank | You’re dosing more often over time | Growing demand (corals/coralline) | Measure daily use for a week; increase dosing slowly |
| KH climbs after switching salt brands | KH rises after water changes | New salt mixes higher dKH | Match new water KH closer to target; reduce dosing |
| High pH + high KH | pH stays elevated, scaling on equipment | Overuse of soda ash/kalkwasser | Reduce/stop high-pH alkalinity sources; stabilize |
| Coral stress after “correcting” numbers | Corals retract after big adjustment | KH changed too fast | Hold steady; slow corrections (≤0.5–1 dKH/day) |
Safety Checklist (Read This Before Dosing)
Don’t chase pH with alkalinity
- If pH is low but KH is normal, look at gas exchange, indoor CO₂, and surface agitation first.
- Raising KH just to raise pH can create long-term instability.
Change one thing at a time
- Adjust alkalinity first, then calcium, then magnesium if needed.
- Wait and observe between changes.
Retest after adjustments and watch livestock
- Recheck KH after dosing changes.
- Watch corals and fish for stress behavior.
- If something looks off, pause changes and confirm your tests.
FAQ (Beginner Reef Keeper Questions)
What is a safe alkalinity (dKH) range for a beginner reef tank?
A stable 8.0–9.0 dKH is a common beginner-friendly target. Many reefs do well anywhere around 7.5–9.5 dKH as long as it’s stable.
How fast can I raise alkalinity without harming corals?
A conservative limit is 0.5–1.0 dKH per day. If corals are already stressed, aim closer to 0.5 dKH/day.
Can I lower alkalinity quickly in a saltwater aquarium?
Fast drops can stress corals. The safest method is usually to reduce/stop alkalinity dosing and let KH fall naturally, using planned water changes only if needed.
Is baking soda safe to increase KH in a reef tank?
Yes, when used correctly. Sodium bicarbonate (baking soda) is a common alkalinity supplement with a gentler pH impact than soda ash. Dose slowly and retest.
What’s the difference between baking soda and soda ash for alkalinity?
Both raise alkalinity. Baking soda has a smaller pH effect; soda ash raises pH more strongly and can be riskier if overdosed or added too quickly.
Why does my alkalinity keep dropping every day?
Most often it’s normal consumption from coral growth and coralline algae, or a dosing schedule that isn’t matching demand. Test daily for several days to measure consumption, then dose that amount consistently.
Call to Action: Make KH Stable (Without Overthinking It)
If you want alkalinity to stop feeling like a guessing game, start logging dKH, dose amounts, and water changes in Reef Buddy. Shrimpy can help you organize your tests, see your KH trendline, and build a simple dosing routine that keeps your reef stable—without chasing numbers.