Why would pH drop suddenly in a saltwater aquarium?

Question

Hey all,
I recently grew mysid shrimp in a plastic tank. The only things in the tanks were pumps, pipes, a few nets, and heaters (no sand or other substrates), and uneaten artemia food. On the second day of the experiment, the pH suddenly dropped: from 8.2 to 7.4, then returned to 8.0 the next day. On this day, the dissolved solids were also at their lowest level of 20.92ppt (usually they were closer to 24ppt).

Is there any reason the pH could have dropped so suddenly? I think the CaCO3 precipitated out somehow (since both pH and TDS dived simultaneously), but cannot for the life of me work out how. I haven't used aquariums very often, but didn't alter anything on the day when everything crashed. I would be grateful of any help.

Thanks

The tests were done with an electronic pH probe. My first thought was that the outlyer was a sampling error, or perhaps contaminated somehow, but other results agree. For example, the salinity being at its lowest, and 18 mysids dying over this 1 day period (I only started with 40, so the amount lost over this day pretty much killed the experiment).

Temperature was also the same as usual on this day. The artemia were not cleaned out, so I was thinking perhaps they seeded a sudden precipitation of CaCO3, then the system returned to equilibrium the next day.

Answer 1

A few thoughts for you:

1) How were you performing these tests? If there were paper strips used rather than liquid reagents (color matching or titration) or a meter to measure the pH, the strips can be "off" if they had been handled improperly or exposed to moisture beforehand. It might have been better to take several readings then use an average - that would have minimized any outliers in your results.

2) The pH in a tank would be correlated to CO2 (a buffering system) - any change in the number of mysids (eggs hatching?)

3) The pH can also lower as a result of wastes accumulating. Was the uneaten artemia removed between days 2 and 3? Any molting by your shrimp?

4) Good old human error - could the second day have been 8.4 instead of 7.4?

Even though #2 and #3 could affect the pH, I doubt that either of these would be what caused your results - typically these would work over a longer period of time to cause an almost tenfold shift. An with the alkalinity that would be needed to get your pH to 8.2, it would be possible to lower it to 7.4 in a short period (absorption of atmospheric CO2, addition on any acid), it's odd that anything that would depress the pH that far would allow it to rise back to almost the original level.

If there were plants in the tank, I would ask if the pH measurements were taken at the same time each day. Since the absorption of CO2 by plants (as a part of photosynthesis) can raise the pH, if the second day the measurement was taken earlier, the lights may not have been on as long, thus more CO2 would have been in the water.

Was there any change of temperature between days? Temperature can affect solubility (of the salts/buffers), thus affecting pH (as well as your TDS).

Answer 2

Salinity can affect ph. You don't mention your specific gravity. Some say it doesn't affect ph but I've had fish only tanks and true reef tanks for over 10 yrs and ph can go up or down if you don't maintain the same level. I keep all three of my tanks at exactly 1.023 and have never had ph issues UNLESS I don't add water lost due to evaporation.