Alkaline aquarium water

Shrimphaus is usually around pH 9.1

I was quite surprised to discover that the Shrimphaus has an equilibrium pH of just over 9.1 which is quite alkaline; over 100x the alkalinity of water at neutral pH of 7.  Chemistry and geology forces this to be the case, discussed below.

Chalk aquifers mean hard water

Cambridgeshire water flows through chalk (calcium carbonate:  CaCO₃) aquifers making the water very hard.  Carbon dioxide from the air dissolves in water converting to a small degree into carbonic acid, which subsequently reacts with and dissolves the chalk, giving both free calcium ions and bicarbonate.

CaCO₃ (s) + CO₂ (aq) + H₂O (l) ⇌ Ca²⁺ (aq) + 2 HCO₃⁻ (aq)

Calcium (and magnesium) ions are components of general hardness (dGH) whilst bicarbonate ions make up the other type of hardness, carbonate hardness (dKH), also known as ‘alkalinity’.  Cambridgeshire water is high in both types of hardness with dGH and dKH both around 17 units.  It is the dKH, the alkalinity of the water, that is driving the high pH.

Atmospheric CO₂, alkalinity and pH equilibrium in water

The bicarbonate system in water is a quite complex balance between atmospheric CO₂, dissolved CO₂, carbonic acid, bicarbonate and carbonate.

In the water, the balance between these different chemical species is determined by pH and the reverse is also true.  This pH dependence on the relative abundance of the different chemical species can be calculated and plotted.

In the above graph, the species listed as H₂CO₃ (nominally carbonic acid), is actually composite carbonic acid:  an equilibrium of about 99.7% dissoved CO₂ and 0.3% carbonic acid. 

The pH of the water turns out to be almost entirely dependent on:

1. carbonate hardness (alkalinity, dKH) – more carbonate hardness increases pH
2. the amount of dissolved CO₂ (composite carbonic acid) – more dissolved CO₂ decreases pH

The carbonate hardness is set by the water used for water changes, for the Shrimphaus that is around 17 dKH.  The amount of dissolved CO₂ in the water is driven to an equilibrium of around 0.5 ppm.  This equilibrium results from the fixed partitioning ratio between atmospheric and dissolved CO₂ – this fixed ratio is known as Henry’s law.

When you do all the maths, at an alkalinity of 17 dKH and 0.5 ppm CO₂, the pH of the water calculates out to 9.07 which is an excellent match for the actual measured pH observation.

Alkaline water impact on plants and animals

Water that is both alkaline (high dKH) and hard (high dGH) has a reputation for making submersed plant growth difficult.  I will say that many types of submersed plants have done quite badly in the Shrimphaus.  The double challenge of low CO₂ (no CO₂ gas injection) and hard water with high pH seems to have restricted the number of species that can thrive.  So far Alternanthera reineckii ‘Mini’ and Helanthium bolivianum ‘Quadricostatus’ have done convincingly well, and some types of cryptocoryne species have also done pretty good.  It is also definitely possible to grow black beard algae under alkaline water conditions – fortunately that has retreated recently as a problem in the Shrimphaus.

Shrimphaus doesn’t have any fish, just bloody mary shrimp.  They seem unfazed by the water conditions and live, eat and breed productively.  Copepods and snails likewise haven’t had any noticeable problems.