Why pH Drifts in Planted Aquariums: KH and Nitrates Explained

Stable pH and transient pH changes

To get started, let’s discuss factors that apply universally to planted tanks.  For simplicity we’ll work under the assumption that aquarium hardscape and substrate is pH inert.  Then there are two main ways pH can change, transiently, usually on a day/night driven cycle, and stably which operates more long-term.  Stable pH is determined by the water’s carbonate hardness (KH).  Transient pH is determined by the level of dissolved carbon dioxide (CO2) in the water.  These work independently of each other:  CO2 changes pH without changing KH, and KH changes the equilibrium pH without changing dissolved CO2.  The overall pH of the water is determined by the stable pH and modified by the transient pH.  To control long‑term stability modifying KH is the best method.  This discussion explains why pH drops in planted aquarium systems and the simple KH–nitrate balance that controls it.

Transient pH is controlled by dissolved CO2

Baseline pH is set by KH and atmospheric CO2 levels.  Transient aspects of pH change from this baseline are controlled by dissolved CO2.  Atmospheric CO2 is involved here as well by setting the equilibrium level of dissolved CO2 (Henry’s law) which is not affected by KH.  From there, adding CO2 lowers pH; removing it raises pH.  At constant KH and atmospheric CO2, a 10× increase in dissolved CO2 corresponds to a 1.0 pH drop (and vice versa).  CO2 increases via respiration (animals, microbes, and plants in the dark) and direct CO2 injection; it decreases via plant photosynthesis in the light.  There is a constant ‘pull’ back toward the air–water equilibrium level of dissolved CO2; larger air–water surface area, stronger surface agitation, active aeration/skimming, and water circulation that renews the water surface speed this return.  None of these factors that change dissolved CO2 change KH or baseline pH; they only move pH around the KH‑set baseline.

Stable pH is set by KH and controlled by nitrate (NO3⁻)

Baseline pH is set by KH and atmospheric CO2 levels as previously mentioned.  That means if your baseline pH is drifting, something is changing the KH of your water.  We’ll start with biological control of KH which long-term is the main driver, then briefly touch on KH-active hardscape / substrate and water changes.

KH active biology

Biology can have huge effects on KH and thereby drive pH changes in an aquarium.  The way biology controls KH is indirect: through nitrate, not by directly changing KH.  Biological processes that make nitrate also make acid (nitrification), reducing KH and pH; biological removal of nitrate removes acid at the same time, raising KH and pH.  There is a chemistry section later for details, but the biological processes involving nitrate are key – nitrate itself isn’t acidic.

Biological producers of nitrate – aquarium water becomes more acidic

Nitrate is produced mainly by nitrifying microbes (often in filters and on surfaces) as they detoxify bioload waste (ammonia/ammonium) from fish, shrimp, and other residents.  That means the more aquarium residents you have, the more nitrate will be produced and more strongly pH will tend to drift downwards.  Nitrate is also produced by decaying organic matter, e.g. mulm, decomposing wood hardscape, etc.  This decomposition will also make pH drift downwards.

Biological removers of nitrate – aquarium water becomes less acidic

This one is pretty simple:  growing plants are the main and best way of removing nitrate.  Plants can be submersed, emersed or floating on the water surface – in all cases growing plants can pull nitrate out of the water column.  Nitrate removal indirectly results in increasing KH so pH will tend to drift up.  The more plants you have and the better they are growing, the stronger this trend will be.

Balancing pH – nitrate production vs nitrate removal

Your aquarium pH will tend to stability when the pH downward drift relating to nitrate production from animals and waste is balanced by the pH upwards drift relating to plant growth.  The usual trend is downwards:  too much livestock, not enough maintenance to clear out organics debris, and not enough actively growing plants.  If your aquarium tends to become more acidic over time, this is probably why.

One for two: you need a lot of plants, minimal waste, and you need an external source of nitrate

Biological production of nitrate is twice as strong at decreasing aquarium pH as nitrate removal by plant growth is at raising aquarium pH.

This is a potentially nasty feature of nitrate controlled KH in a planted aquarium and explains the typical downward pH drift over time.  The chemistry section explains in detail why this is the case, but there are two main take-home messages for the aquarist:

1. To keep KH (and therefore pH) constant using biology alone in a well‑oxygenated tank, plants must remove about 2× as much nitrate as your fish/shrimp/organic decay produce.  In practice that means a clean tank and lots of fast‑growing, healthy plants.
2. Because your tank biologically produces only half as much nitrate as plants would need to remove to keep KH (and pH) stable, the “other half” has to come from somewhere else.  That’s usually nitrate normally found in tap water and/or a small nitrate dose (e.g. potassium nitrate – KNO3) in fertiliser that you provide your tank.  Plants remove that extra nitrate and, with it, the extra acid needed to keep KH and pH steady.

Without an external source of nitrate, your tank will trend to become more acidic over time.

Note about KH active substrate and hardscape

KH‑active hardscape (usually limestone, shell, or coral) dissolves in the presence of the small amount of carbonic acid made from CO2 in water which adds bicarbonate (HCO3⁻) thereby nudging KH and baseline pH up.  The effect is more pronounced in low‑KH water and tends to taper off gradually over time.  KH‑active substrates (aquasoils/“active soils”) usually do the opposite:  they pull KH/pH down either directly by adding acid (H⁺), or indirectly by adding ammonium (NH4⁺).  This effect can initially be quite aggressive but fades over the course of few weeks to months as the substrate becomes “exhausted”.  Once you account for any KH‑active rock or soil, remaining long‑term changing pH trends are driven by biological effects on KH.

Note about water changes

Water changes are great for stability, but not because they “remove nitrate.”  Nitrate didn’t cause KH to drop; the nitrification that produced that nitrate spent the KH, and the nitrate just tells you that already happened.  What a water change really does is reset the tank toward your source‑water KH.  If your supply has some KH, you add buffer and pull baseline pH back toward that of the supply; if it also has some nitrate, plants can strip that “extra” nitrate and raise KH biologically.  If your supply is low KH and low nitrate, a water change alone won’t hold pH steady, so add a bit of KH buffering (1 dKH floor is plenty) and/or a little nitrate dosing.  Bottom line: change water regularly to keep organics down and to reset KH toward known values; don’t chase “zero nitrate”, you need some for plant growth to resist acidification.

Chemistry section (optional)

TL;DR

• Nitrate production (nitrification) spends KH and lowers equilibrium pH.
• Nitrate removal (plants/denitrification) returns KH and raises equilibrium pH.
• Dosed or imported nitrate is KH‑neutral until removed (by plants/denitrification).
• Each 1 ppm NO3⁻ produced results in −1.612 ppm KH; each 1 ppm NO3⁻ removed results in +0.806 ppm KH
  • NO3⁻ in ppm as NO3⁻; KH in ppm as CaCO3 equivalents.

In the beginning:  production of ammonium (NH4⁺)

Ammonium is the usual result of fish/shrimp nitrogen metabolism and of bacterial decay of organic matter.  Two major biochemical routes dominate but the end result is the same:  the biological nitrogen is converted to NH4⁺ and released into the water column.  This first step does not change KH (or pH).

Hydrolytic deamidation (glutaminase)

• L‑glutamine + H2O → L‑glutamate⁻ + NH4⁺

Produces ammonium; no net proton generated, so KH (alkalinity) is unchanged.

Oxidative deamination (glutamate dehydrogenase; NADH re‑oxidized by O2)

Deamination proceeds in two connected steps:

• L‑glutamate⁻ + NAD⁺ + H2O → α‑ketoglutarate²⁻ + NH4⁺ + NADH + H⁺
• NADH + H⁺ + 1/2 O2 → NAD⁺ + H2O

Net (summing and canceling NAD species, H⁺, and H2O):

• L‑glutamate⁻ + 1/2 O2 → α‑ketoglutarate²⁻ + NH4⁺

Produces ammonium; no net proton generated, so KH (alkalinity) is unchanged.

Nitrification (NH4⁺ → NO3⁻): where KH is lost (pH goes down)

This is the part where your tank gradually becomes more acidic over time.  Ammonium is converted to nitrate.  That’s a good thing:  ammonium can interconvert with ammonia, and ammonia is highly toxic.  A healthy, well‑oxygenated aquarium supports robust nitrifying biofilms (often in the filter) that eliminate ammonia by oxidizing ammonium to nitrate.  Having some nitrate around is normal—plants use it, a lot of tap water contains it, and it’s the “N” in NPK fertilisers.  Nitrate at typical aquarium levels generally won’t hurt your livestock.

The catch is… converting ammonium to nitrate creates acid.

• NH4⁺ + 2 O2 → NO3⁻ + 2 H⁺ + H2O

For every unit of NH4⁺ produced from animal waste and decaying organic matter, two H⁺ (protons) are produced.  Those protons neutralise bicarbonate:

• 2 H⁺ + 2 HCO3⁻ → 2 H2CO3 ⇌ 2 CO2 + 2 H2O

The CO2 then degasses to the room.  Net effect: each unit of NH4⁺ nitrified removes two units of bicarbonate, decreasing carbonate alkalinity (KH) and pulling the equilibrium pH down (after CO2 re‑equilibrates).

Nitrate removal by plants: adding back KH (pH goes up)

Plants are the big reversers of KH loss in an aquarium.  They need nitrogen for growth, and in planted tanks nitrate (NO3⁻) is usually their main inorganic N source.  Most plant nitrate transporters also bring in one proton with each nitrate (H⁺/NO3⁻ symport), so plants effectively remove one H⁺ from the water per NO3⁻ they take up.

• NO3⁻ (aq) + H⁺ (aq) → “new plant N” (net effect: −1 H⁺ from the water)

Where does that H⁺ come from?  From the carbonate buffering system in the water that is in equilibrium with room air.

• CO2 (g) + H2O ⇌ H2CO3 → H⁺ + HCO3⁻

When plants remove H⁺ with NO3⁻ uptake, they force the carbonate system to run the acidification reaction in reverse: CO2 from the air is hydrated to make H⁺ and HCO3⁻, the plant uses the H⁺ with NO3⁻, and the HCO3⁻ left behind raises KH and nudges equilibrium pH back up.

Note about microbial denitrification

Denitrification needs very low oxygen, so it mostly happens in clogged filter cores or in packed mulm/deep, compacted substrate.  Each NO3⁻ reduced to N2 removes one H⁺ from the water (same KH “refund” as plant uptake, different chemistry).  In a well‑run, well‑oxygenated tank it’s a small contribution, and it’s better to rely on healthy plants for nitrate control.

The maths overall

• Each 1 ppm NO3⁻ produced by nitrification consumes 1.612 ppm KH (as CaCO3), a decrease of 0.09 dKH.
• Each 1 ppm NO3⁻ removed by plants (or by denitrification) adds 0.806 ppm KH, an increase of 0.045 dKH.