Regular and substantial water changes for an aquarium are a good idea.  With EI fertiliser dosing the built-in assumption is that at least 50% of the water will be changed out every week to prevent a build-up of excess fertilisers.  Both the Fireplace Aquarium and the Shrimphaus follow this maintenance schedule, although lately I have been doing around 75% water changes to better remove organic particulate debris from the Fireplace Aquarium and to remove salts from the Shrimphaus.

Pre/post water change parameters

Fireplace AquariumShrimphaus
pHalkalinity (ppm CaCO3)pHalkalinity (ppm CaCO3)
before water change7.852649.01342
immediately after water change7.962768.67306
next day7.752799.03309

Water change in the Fireplace Aquarium

Since the Fireplace Aquarium has a closed lid, there isn’t much evaporation and other than what is metabolised/catabolised by the inhabitants water components stay pretty stable.  The pH of the Fireplace Aquarium goes up following a water change, because the Fireplace Aquarium has injected CO2 gas which lowers pH and changing the water removes this extra dissolved CO2 which will need to be regenerated gradually by the CO2 injection system.  I haven’t been very aggressive on the CO2 injection lately, only going to around 10 ppm, so the effect on pH is modest.  The alkalinity of the Fireplace Aquarium doesn’t really change much including following a water change, although the bioload from the fish will tend to decrease alkalinity over time.  CO2 injection lowers pH but does not change alkalinity, so that’s not a factor.

Water change in the Shrimphaus

Shrimphaus is an open-top high surface flow setup and loses about 1 litre water per week to evaporation which increases the carbonate hardness and in due course the pH of the water as well.  A water change does a good job of resetting the alkalinity.  In the water change above, 10 litres of tap water was added to 7 litres of residual not-changed-out water.  The maths on this match up with the observed values really well:  the tap water has an alkalinity of 285 ppm CaCO3 resulting in a weighted average post-water-change of (10 x 285 + 7 x 342) / 17 = 308 ppm CaCO3.  The pH of Shrimphaus post-water-change is artificially low because Cambridgeshire tap water is pressurised and has high levels of dissolved CO2 gas.  After this excess gas equilibrates out into the environment, which takes around 4 hours in the Shrimphaus, the pH returns to the expected equilibrium value.

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