pH Question...Imagine That

[rooster426]

I know the answer(s) to this questions are probably sitting on this forum right now, but I'm asking anyway: This is a question posed to me by a friend: In a mature reef tank (125 gallons?) with all major parameters seemingly in normal ranges, what would/could cause the pH to be consistently low? It seems to be more than a night time/daytime shift. They can raise the pH but it eventually drops to about 7.8-7.9. It's a constant swinging up and down struggle. They swear that the alkalinity, calcium, salinity, temp, are good. They are not using a calcium reactor, but dosing with a two-part additive. Maintenance seems good in that there is no dirty filter media or over-feeding. My first thoughts were poor aeration and resulting co2 imbalance? My more confused reaction/thought was the relationship between carbonate, bicarbonate could be to blame for the inconsistent pH. I know the relationship goes one way - with pH altering the concentration of both, but does it work the other way - with the relative concentration of either carbonate or bicarbonate effecting the pH? I'm not sure that's even a logical question? Any help would be appreciated. Thanks

[Boomer]

Rooster it is a CO2 issue. At night plants give off CO2 rather than take it up, which lowers the evening pH. In the morning they start to pull out the CO2, which raises it back up. pH should not be raised using buffer, buffers are for Alk not pH. Only in special circumstance can buffers like "Washing Soda/ Soda As/Baked Baking Soda can be used, when there is both a low pH and Alk. Although Kalk is really not a buffer, Kalk reacts with CO2 raising the pH. This is what is often used as a nigh drip to keep the pH up from excessive CO2.

One of the issues with many calcium reactors is low pH due to the excessive CO2 left behind. This is the purpose behind Dual calcium reactors, which correct this CO2 issue of the most part. But this does not seem to be their issue.

What controls the pH is the ratio of CO2 to bicarbonates and carbonates. When CO2 is added or removed it cause the pH to drop. How high the Alk is, is irrelevant. Meaning if the Alk was 2 dKH or 20 dkH, if CO2 increase the pH will drop and if CO2 leaves the system the pH will rise. It is normal for tanks to have a nigh day shift in pH, often 0.2 pH units between day and night. Many reefs are the same.

Here is a good review on CO2 and pH

Low pH: Causes and Cures
http://reefkeeping.com/issues/2004-09/rhf/index.htm

[rick s]

Rooster,

Ask him if his pH goes up during Spring and Fall when he has his windows open.

I also was plagued by low pH. Mine started dipping into the 7.7's and my corals did not like it.

Like Boomer mentioned, I now dose all my replacement water, as kalk, at night when pH is typically at its lowest. I dose kalk from the moment the lights go out until they come back on. Doing this has raised my daily lows by .15 - .2.

Between having all natural gas appliances, a fairly tight home, and 2 cats; it's amazing how much CO2 I have in my house.

[rooster426]

Thanks for the help. CO2 seems the likely culprit to me. I read the article - very helpful. It's amazing the information that's on this website. If only my brain could make sense of it all...I guess I'll keep trying.