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Water chemistry

Water chemistry, condensed

pH, KH, GH, ammonia, nitrate — what each parameter actually represents in solution, which ones move on a daily basis, and which test kits earn the shelf space.

Water chemistry, condensed

The hobby talks about water chemistry like it's a single number — "what's your pH?" — when it is actually six parameters that interact along well-understood equilibria. This guide is the minimum mental model: what each measurement represents physically, how they relate, and which ones move daily versus monthly. The numbers you should care about week to week are fewer than most aquarium starter kits suggest.

Reference chart — pH as a function of KH and dissolved CO₂. The curves show that adding CO₂ at any given KH lowers pH along a predictable equilibrium
Reference chart — pH as a function of KH and dissolved CO₂. The curves show that adding CO₂ at any given KH lowers pH along a predictable equilibrium

The chart above is the carbonate equilibrium visualized. Both forms of the equation shown — the APHA-derived pH = 6.37 + log₁₀(12.84 × KH / CO₂) and the hobby-simplified pH = 7 + log₁₀(3 × KH / CO₂) — produce essentially identical values in the planted-tank pH range.

The carbonate system is the backbone

Nearly everything else flows from the carbonate-bicarbonate equilibrium in solution:

$$ \text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{HCO}_3^- + \text{H}^+ \rightleftharpoons \text{CO}_3^{2-} + 2\text{H}^+ $$

This single chain of reactions sets pH, controls how much CO₂ dissolves, buffers acid additions, and limits how dramatically pH can swing under photosynthetic load. Stumm & Morgan's Aquatic Chemistry (3rd ed., 1996) remains the canonical reference; for aquarium-relevant simplifications, Walstad (2013) is more practical.

The key shorthand: in the pH range typical of freshwater aquaria (6.5–8.0), most carbon is in bicarbonate (HCO₃⁻) form, with smaller fractions as dissolved CO₂ and carbonate. Bicarbonate is what your KH test measures.

pH is a state variable, not a target

pH is the negative log of hydrogen ion activity. Two consequences for hobbyists:

  1. It's a ratio scale. pH 6 is ten times more acidic than pH 7, not "one unit." A 0.5-unit drop overnight is biologically significant; a 0.1-unit drift is noise.
  2. It is set by the chemistry above it, not adjusted directly. Adding "pH down" without dropping KH first is fighting the buffer system — you'll consume the additive and the pH will rebound within hours.

Fish and plants tolerate ranges, not pin-point values. Caridina cantonensis (Bee shrimp) tolerate 5.5–6.5; Tropheus cichlids need 8.0–9.0. The right question is never "what's my pH" but "does my pH range overlap the species' range with margin to spare?" Use the water chemistry calculator to convert KH+CO₂ to expected pH.

Daily fluctuation in planted tanks with CO₂ injection is expected: pH drops 0.5–1.0 units when CO₂ turns on at dawn, recovers when it shuts off at dusk. This is the carbonate equilibrium responding to gas input, not equipment failure.

Hardness: GH and KH measure different things

GH (general hardness): total Ca²⁺ + Mg²⁺ + other divalent cations. Important for shrimp molting (calcium for the new exoskeleton, Mg for the enzymatic processes that soften the old one) and for hard-water fish (Rift Lake cichlids, livebearers). Target ranges by species — soft water fish prefer GH 2–6, hard-water species need 12+.

KH (carbonate hardness): the bicarbonate buffer capacity. Determines how fast pH responds to CO₂, organic acids, and water changes. KH below 3 °dKH makes pH unstable; CO₂ injection can crash pH within hours in unbuffered water. KH above 6 °dKH is unnecessarily restrictive for low-tech tanks.

Confusion arises because both are reported in "degrees" (°dGH, °dKH) despite measuring chemically unrelated species. Test kits use different titrant chemistries — don't conflate readings.

The nitrogen cascade is the diagnostic

Ammonia, nitrite, and nitrate are the three measurements that tell you whether your biofilter is working. The biology behind them is detailed in the nitrogen cycle guide; the rules of thumb:

  • Ammonia and nitrite must read zero in any tank with fish. Anything else is an acute health emergency.
  • Nitrate is a fertilizer in planted tanks. Zero nitrate for months usually means your kit is unreliable or your plants are starving. The Estimative Index methodology (Barr, 2005) deliberately maintains 10–30 mg/L nitrate via dosing.
  • High nitrate with no plants indicates insufficient water change frequency. Set a calendar.

Note: ammonia toxicity is governed by free NH₃, which scales with pH. At pH 7.0 and 25 °C, 0.5 mg/L total ammonia is mostly NH₄⁺ (low toxicity). At pH 8.5 the same total is 5–8% NH₃ — already in the acute-toxicity range for most tropicals (Erickson, Water Research, 1985; USEPA 1999 update).

What you actually need to test, by stage

Weeks 1–6 (cycling): ammonia, nitrite, nitrate, every 2–3 days. KH and pH weekly to confirm buffer stability. Read the cycling guide for what numbers to expect each week.

Months 2–6 (establishing): nitrate weekly to calibrate water change frequency. KH and GH once a month to track substrate aging — active soils consume KH over time. pH spot-checks if you're running CO₂.

Steady state (6+ months): nitrate before each water change, KH monthly, ammonia/nitrite only if something looks off (a fish behaves strangely, plants suddenly melt). The mature system doesn't need daily testing; it needs you to maintain the inputs (food, light, water changes) that kept it stable in the first place.

What not to spend money on

  • "Total alkalinity" strips — same as KH but lower precision.
  • Iron, copper, phosphate kits in your first year — useful for diagnosing specific plant deficiencies later, not as baseline monitoring.
  • TDS pens before you understand KH/GH — TDS measures total dissolved solids, which lumps useful and useless together. Once you know your GH, TDS becomes a quick proxy for "did my RO membrane fail?" — but until then it's an unmoored number.

A reliable liquid ammonia/nitrite/nitrate kit (API, Salifert) plus a GH/KH test will diagnose 90% of beginner problems. Use the tools hub for calculators that turn raw readings into actionable adjustments.

References

  • Stumm, W., & Morgan, J. J. (1996). Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters (3rd ed.). Wiley-Interscience.
  • Walstad, D. (2013). Ecology of the Planted Aquarium (3rd ed.). Echinodorus Publishing.
  • Erickson, R. J. (1985). An evaluation of mathematical models for the effects of pH and temperature on ammonia toxicity to aquatic organisms. Water Research, 19(8), 1047–1058.
  • USEPA (1999). 1999 Update of ambient water quality criteria for ammonia. EPA-822-R-99-014.
  • Boyd, C. E. (1998). Water Quality in Pond Aquaculture. Auburn University.
  • Barr, T. (2005). The Estimative Index. The Barr Report, online publication.

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