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Re: HCL dosing

Len wrote:

I recently purchased some 20% HCL and have been dosing a small amount every
night trying to lower the ph of my tank.  It is a 75g tank with tap water
that is about 8.25 ph, 100+ ppm GH and 120+ ppm KH.  For the past 4 nights
I've added 15-20 drops of HCL to the sump.  Every night I measure the ph
before putting in the HCL and about 2 hours afterwards.  Every evening
before putting in the HCL the ph has been up to or very close to 8.25, but
after adding the HCL the ph drops to about 8.  I'm concerned about dropping
the ph to quickly, but I didn't expect the ph to keep going back up so much.
Am I doing something wrong or is it the buffering of the hard water causing
this?  Should I put in more HCL?  Any suggestions would be appreciated.
I've been very cautious doing this and so far none of the fish or plants
show any signs of problems.

I reply:

The reason the pH goes back up is because when you add HCl to your tank the
acid reacts with the carbonate in your water and this releases CO2 into your
water. Your pH drops mostly because of the CO2. After a while the CO2 gasses
off and the pH rises again. If you added acid enough days in a row
eventually enough of the carbonate would be consumed to the point where your
pH would significantly drop. By that time it would be time for another water
change and you would be back to square one.

There is probably no good reason for you to lower your pH but if you are
determined to do this then you should add the acid to your change water,
wait for the pH to stabilize then add the change water to your tank. Over
time with water changes your pH in your tank will come to match the pH of
your incoming change water. Adding HCl also has the effect of adding
chloride ions to your tank. I don't really know if this is better than
having a high pH or not. I rather suspect it is a trade off that amounts to
about the same thing as far as your fish are concerned.