pH advice please

[mindi]

I have just had an unexpected pH excursion and am at a loss to understand why and more, how to respond. My mixed reef has never gone outside 8.1-8.3 in years and 2 days ago it dropped to 7.6, recovered yesterday to 8.0, and today is at 7.6. All measured on a newly calibrated pH meter. My Alk is 3.4meq/l and my Calcium is 400 both on Hanna.
I have read Randy on pH but dont think I can respond with Limewater as I will push alkalinity too high..?

The only strange thing I have done recently, and it seems immediately to be the problem...I removed a large rock covered in pulsing Xenia which was way out of control. I couldnt think how to kill it so put it in the sink and poured vinegar over it. I then poured RO water over it to wash away the vinegar. Is the explanation that this must be the cause..? I thought I flushed it really well....and how do I respond..??.My Elegance looks unhappy and a BTA has gone into hiding.

ps... I have just put two airstones in the tank in the hope that they might strip out some CO2. I did notice it lowest in the morning but that would be normal. I still feel I need to neutralise the vinegar experiment.

[bertoni]

The rock might have some organic debris or some toxins might have gotten back into the tank. I would try some 15-20% water changes and run some fresh carbon. Hopefully, the corals will recover quickly.

The pH excursion is a bit strange. My first guess is that it's a measurement problem. Dropping the pH to 7.6 like that would require a lot of carbon dioxide in the air, assuming the decay rate in the tank is normal. I probably would keep watching it for a while.

[mindi]

Yes thats about how I feel. The rock was only out of the tank a few minutes so cant see what opportunity I gave anything except the vinegar...maybe the aeration will help.

[mindi]

Quote RHF....." Remove a cup of tank water and measure the pH. Then aerate it for an hour with an airstone using outside air. The pH should rise if the pH is unusually low for the measured alkalinity, as in Figure 3 (if it does not rise, most likely one of the measurements (pH or alkalinity) is in error). Then repeat the same experiment on a new cup of water using inside air. If the pH rises there too, then the aquarium pH will rise with more aeration because it is only the aquarium that contains excess carbon dioxide. If the pH does not rise inside (or rises very little), then the inside air contains excess CO2, and more aeration with that same air will not solve the low pH problem (although aeration with fresher air should)."

This is a really good test and gave me unexpected but repeatable results. Very easy to do...in my case the imagined "bad" air inside raised the pH in the sample by the same amount as "fresh" air outside proving that CO2 over-saturation is restricted to the tank rather than the house air. Very interesting..I was blaming poor ventilation...end of winter, house almost entirely closed up etc etc... but that was wrong. Great test Randy...Thanks..!