Low alkalinity Shrimphaus

Despite all the advice to the contrary, I tried out some Caridina shrimp in the tapwater-based Shrimphaus.  This did not go well and after 3 weeks there was only one brave survivor left.  The usual way to keep soft water shrimp is to start with reverse-osmosis (RO) water which is selectively remineralised mostly to restore calcium and magnesium (general hardness – GH).  Alkalinity (KH) is usually kept pretty low.  The problem with this approach is that RO equipment can be expensive, takes up a good amount of space, and wastes a lot of water.  There is an interesting RO water supplier, Spotless Water, that has RO water vending stations distributed around England, but there aren’t any in the Cambridgeshire area.  Having RO water shipped in directly isn’t very practical.

What do to about that?

Remove excess alkalinity with acid

It is (relatively) straightforward to remove alkalinity from high KH water by adding an appropriate amount of a strong acid.  Acidifying alkaline water converts bicarbonate to carbonic acid where the carbonic acid subsequently interconverts to CO₂ which is outgassed to atmosphere.  I did some experimentation with this approach in the Shrimphaus before by adding 1N HCl twice per day.  There are some downsides:  adding the HCl turns out to be a lot of work, I didn’t notice that the released CO₂ had any materially beneficial effect on plant growth and the pH of the water shifts quite dramatically starting high right after a water change and ending up low before the next water change, with actually quite jerky steps of pH along the way.  The neocaridina shrimp seem non-perturbed by the whole process, but I suspect this isn’t going to work for the more sensitive caridina species.

Pretreat and equilibrate water to remove KH

I am trying a new approach by pre-treating the water to remove the alkalinity before a water change rather than with a gradual stepwise approach.  Tapwater has dechlorinator added and then is equilibrated with environment in a bucket using an airstone overnight.  I then measure the water alkalinity and add an amount of 1N HCl suitable to bring the KH down to 30 ppm.  After another overnight equilibration using the airstone to ensure all the excess carbonic acid has been removed we’re ready to use the treated water in a Shrimphaus water change.

In the first go at this, the tapwater had a KH of 252 ppm CaCO₃ equivalents and had a pH of 7.95.  After adding HCl and equilibration, the water had a KH of 33 ppm with a pH around 7.25.  I did the waterchanging in the Shrimphaus over several days and a number of iterations to not change the water parameters too suddenly.  At the end of the process the Shrimphaus water had a KH of 36 ppm and a pH of 7.5.  Aiming for KH in the 30-35 ppm range works out to a dKH of between 1.5 – 2.0, for which there is some evidence both neocaridina and caridina shrimp can do well.  We’ll see what effect (if any) the reduced alkalinity has on the plants.

Bring alkalinity down to 15 ppm 

Having done this successfully for a while now, I now routinely bring the alkalinity down to 15 ppm instead of the 30 ppm described above.  I think that works a little better for the caridina shrimp which are now breeding well.  I have also lately been skipping the dechlorinator step on the ground that Cambridgeshire tap water is not treated with chloramines and the chlorine will outgas fully on its own overnight.  DO NOT SKIP THE DECHLORINATOR STEP.

The bromophenol blue titration is very easy to see with 15 ppm water:  pale blue to start, dusky rose after 1 drop of 30 mM HCl, pale yellow after 2 drops.  i find that between water changes the alkalinity in the Shrimphaus slowly drifts upwards which I attribute to plant-based metabolism of nitrate.  I do weekly water changes of least 50% of water volume, usually more, swapping in aged, equilibrated, chlorine-neutralised 15 ppm alkalinity water and by the end of the week the alkalinity has typically risen to 21 ppm CaCO₃ equivalents or so.

Acid does not remove GH

The acid removes the KH but does not remove the GH.  Ordinary Cambridgeshire tap water is well-saturated for both calcium ions and bicarbonate with dGH and dKH both around 17.  This means the Shrimphaus is venturing into relatively unexplored territory of having simultaneously high GH and low KH.  I haven’t noticed any obvious problems with either moulting or breeding for either caridina or neocaridina species.  I suspect the idea that GH can be ‘too high’ for shrimp is a myth.

Brewers add acid to lower the pH of brewing beer

Turns out adding acid to lower the alkalinity of water is a very old trick – beer brewers have been doing this for hundreds of years.  Although most will use lactic acid to (somewhat cheekily) maintain compliance with Bavarian beer purity laws, if you’re not too worried about that, then food-grade HCl works perfectly well.