What's a typical alk consumption rate?

For a mixed-reef SPS-dominant tank, 0.5–1.5 dKH/day at full coral load. Softie-only tanks consume 0.1–0.3 dKH/day. New tanks with little coral see ~0.1 dKH/day or less.

Why does my Ca drop look normal but alk crashes?

Either kalk is dosing alk in addition to your two-part, your alk test reads higher than reality, or you're under-dosing the alk side. Re-check with a fresh reagent kit and a known reference solution (Hanna or BRS calibration standards).

Can I rebalance by just bumping one side?

Short-term yes, long-term no. The two parts (whatever brand) are formulated to dose at the same volume — bumping one creates a different imbalance (free chloride or carbonate buildup). Better to find the root cause: doser leak, test-kit drift, or unaccounted supplementation.

Calculator

Two-Part Imbalance Diagnostic

Name: Two-Part Imbalance Diagnostic
Author: reefcalcs

Two-part dosing (alk + Ca separately) only works when both sides keep pace with coral consumption. Test alk and Ca on day 0, dose normally for a week, retest. The drop ratio should be 1 dKH alk : 7.5 ppm Ca — the stoichiometric requirement to build CaCO₃ skeleton. Imbalance shows up as either alk dropping but Ca holding (under-dosing Ca) or Ca dropping faster than alk (kalk overlap or leaky doser).

JavaScript is disabled in your browser, so the interactive calculator below won't run. The math, formula, and worked example are all written out further down this page under “How this is calculated.” Enable JS (or open this page in another browser) to use the live calculator.

Whether you compute by hand or in the calculator, the same caveat applies: verify against your test kit and the linked primary source before dosing. reefcalcs is reference information, not veterinary or engineering advice.

Two-part imbalance check

Test alk and Ca on day 0, dose normally, retest on day N. Enter both readings below to verify your two-part is balanced.

Days between

Alk day 0 (dKH)

Alk day N (dKH)

Ca day 0 (ppm)

Ca day N (ppm)

Alk drop / day

0.143dKH

Ca drop / day

1.1ppm

Expected Ca drop

7.5ppm

7.5 ppm Ca per dKH alk consumed.

Imbalance ratio

1.07

1.0 = perfectly balanced.

BALANCED — keep your current dosing

• Two-part is balanced. Ca and alk are dropping at the expected stoichiometric ratio.

Stoichiometry: every dKH of alkalinity consumed by coral skeleton growth pulls ~7.5 ppm of calcium with it (CaCO₃ formula weight). Imbalance signals over- or under-dosing.

How this is calculated

Stoichiometry of CaCO₃:
  1 dKH alkalinity = 7.5 ppm Ca consumed

Per-day drops:
  alk_drop_per_day = (alk_0 - alk_N) / days_between
  ca_drop_per_day  = (ca_0 - ca_N) / days_between

Expected Ca drop over the period:
  expected_Ca_drop = alk_drop_total × 7.5

Imbalance ratio = observed_Ca_drop / expected_Ca_drop
  0.8 – 1.2 → balanced
  < 0.8    → Ca under-dosed (or alk over-tested)
  > 1.2    → Ca over-consumed (kalk, leak, precipitation)

Why 7.5 ppm Ca per dKH? Calcium carbonate formula is CaCO₃ — 40 g/mol of Ca, 100 g/mol of CaCO₃. Each mole of carbonate locked into skeleton takes one mole of calcium. Converting through alkalinity equivalents and ppm: 1 dKH = 17.86 ppm CaCO₃ = 7.14 ppm Ca. The 7.5 rounding accounts for trace strontium / magnesium uptake.

If you get no measurable consumption, your test window is too short. Drop tests resolve to 0.5 dKH; reading the same number two days apart doesn't mean consumption is zero. Use a 5–7 day window for reliable signal.

FAQ

What's a typical alk consumption rate?: For a mixed-reef SPS-dominant tank, 0.5–1.5 dKH/day at full coral load. Softie-only tanks consume 0.1–0.3 dKH/day. New tanks with little coral see ~0.1 dKH/day or less.
Why does my Ca drop look normal but alk crashes?: Either kalk is dosing alk in addition to your two-part, your alk test reads higher than reality, or you're under-dosing the alk side. Re-check with a fresh reagent kit and a known reference solution (Hanna or BRS calibration standards).
Can I rebalance by just bumping one side?: Short-term yes, long-term no. The two parts (whatever brand) are formulated to dose at the same volume — bumping one creates a different imbalance (free chloride or carbonate buildup). Better to find the root cause: doser leak, test-kit drift, or unaccounted supplementation.

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<iframe
  src="https://reefcalcs.com/embed/two-part-imbalance/"
  width="100%"
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Sources & references

01
Randy Holmes-Farley — Two-part calcium and alkalinity additive system
https://reefs.com/magazine/aquarium-chemistry-a-homemade-two-part-calcium-and-alkalinity-additive-system/
02
Randy Holmes-Farley — Calcium and alkalinity in reef tanks
http://reefkeeping.com/issues/2002-09/rhf/feature/index.php
03
Hamza's Reef — Two-part imbalance calculator (methodology reference)
https://www.hamzasreef.com/Contents/Calculators/

Last reviewed 2026-05-25