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Maives
12/17/2010, 07:07 PM
I have a new tank that has a pH on the low side. Live rock from an established tank and live sand is minimizing the cycling. A low of 7.67 and a high of 7.95 without adding kalk or carbonate. Both kalk or carbonate, as expected, raised the pH transiently to ~8.1, but then it wants to re-equilibrate on a low pH. I am measuring pH with a PinPoint pH probe and have verified the pH using different buffers. Using SeaChem Reefsalts, my Ca is about 540 ppm and the KH is about 16 dkh. I have also opened my windows which made some improvement and also aerated the sump (uncertain if this improved the pH). I am beginning to think the high Calcium is the culprit. I don't have a Mg kit, so maybe the SeaChem is loaded with Mg and this is keeping Ca supersaturated. Any thoughts? I really expected a snow storm with the high Ca and KH I am measuring, but it hasn't happened yet. Corals are happy, however and are showing good growth under the LEDs. But I really want to get the pH to about 8.2 +/- 0.1

Percula9
12/18/2010, 12:21 AM
Try a water change with a salt with lower levels of calcium and carbonate. Are you supplementing the alk? If your live stock is doing well I wouldn't worry about the pH that much.

noobtothereef
12/18/2010, 01:02 AM
carbon dioxide.... why do you have your calc and alk that high? with a kh of 16 i would think any stony corals would be suffering, especially sps

Maives
12/18/2010, 09:29 AM
carbon dioxide.... why do you have your calc and alk that high? with a kh of 16 i would think any stony corals would be suffering, especially sps

As pointed out this is a new tank. I used SeaChem Reef Salts made with RO/DI. So that is the first reason the Ca and alk is high. Then in trying to boost the pH, I have been adding kalk and/or baked baking soda (i.e., Na carbonate), which increases the Ca and alk even higher. Animals in the tank are doing fine and corals are growing with nice polyp extension. Somewhere I read that high Ca and high alk can lead to low pH. I am trying to find that article again.

mrstihl
12/18/2010, 09:54 AM
Do a series of smaller water changes with regular instant ocean daily until it gets where you want it to be.

HighlandReefer
12/18/2010, 11:14 AM
As pointed out this is a new tank. I used SeaChem Reef Salts made with RO/DI. So that is the first reason the Ca and alk is high. Then in trying to boost the pH, I have been adding kalk and/or baked baking soda (i.e., Na carbonate), which increases the Ca and alk even higher. Animals in the tank are doing fine and corals are growing with nice polyp extension. Somewhere I read that high Ca and high alk can lead to low pH. I am trying to find that article again.

Provided your alk level is within the recommend guidlines, the factor that causes low pH is the amount of CO2 saturated in your tank water. In many modern homes which are tight, the CO2 level is high and this extra CO2 will dissolve into your tank water lowering the pH. As long as your tank pH remains between 7.8 - 8.5 your are fine.

Using high pH alk supplements will increase your tank pH a bit, but you add them to maintain your alk level. If you add these alk buffers to maintain your pH it will drive your alk level too high in many cases. ;)

Randy shows how alk and CO2 levels affect pH in this article:

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


From this article:



http://reefkeeping.com/issues/2004-09/rhf/images/Figure1.jpg
Figure 1. The relationship between alkalinity and pH for seawater equilibrated with air containing normal and elevated carbon dioxide levels. The green dot shows natural seawater equilibrated with normal air, and the curves reflect the result that would be obtained if the alkalinity were artificially raised or lowered.

bertoni
12/18/2010, 02:38 PM
I agree that carbon dioxide is the most likely cause of the problem. Measurement errors are common, too. You could try aerating a cup of water outside for 3 hours, and checking the pH. If it hasn't risen about 8.1 or so, the measurement likely is wrong.

I'd let the alkalinity drop to at least 11 dKH, and work from there. Kalk will help the most with high carbon dioxide, so I'd continue that after the dKH is down. Some people are successful running a skimmer air line outside the house, although that can be tricky.

mm949
12/18/2010, 04:09 PM
i also agree with co2 build-up.....increasing the surface aggitation in the aquarium and sump should help with gas exchange....if using glass lids, remove half the glass, and run some high grade carbon for a few weeks

Maives
12/18/2010, 04:32 PM
Perhaps a little description of system would help. This is a 42 gal acrylic eurobraced cube that is open. DIY LED with 40 3W Crees, 50% blues and whites. Previously no skimmer because I wanted to get a little bit of algae growth for my Potter's angel and help with water quality. System maintained at 78 +/- 0.5 deg F by the Apex. Return flows into a sock. Sump pump is a QO 5000, so more than adequate flow and surface agitation. Currently a mixture of small SPS and LPS, a clown and possum wrasse. Excess acid production should not be a factor due to bioload, but lack of algae may be keeping the CO2 high.

Skimmer has been hooked up today. Before I hooked up the skimmer (Aqua C Remora), I noticed the Ca has fallen, perhaps due to precipitation with the added carbonate. This seems to have helped as pH is currently 7.97 and holding. The big question is what will happen when lights go out. Alk is currently 12 dkh. I have always been suspicious of the high calcium from the SeaChem Reef salts. Calcium peaked at about 540 ppm now 480 ppm.

bertoni
12/18/2010, 04:54 PM
The calcium level is rather high, but it doesn't seem to cause problems.

The skimmer should help with aeration. Another issue can be the buildup of a surface film. I'd check that, too, and point some powerheads at the surface to get a good rippling going.

You are correct that the saltwater will equilibrate to a lower pH over time, if the kalk dose is stopped. Kalk and other high-pH supplements consume carbon dioxide from the water. Aeration will bring it back in.

Maives
12/18/2010, 05:31 PM
Surface agitation definitely is not it as there is lot's of agitation from 2 3/4" outlets directed right at the surface. The in-tank vs outdoor test yielded pH 7.96 vs pH 8.04. However you need to temperature correct because the tank is at 25 C and the outdoor sample was at 13 C, therefore difference is negligible.

bertoni
12/18/2010, 07:45 PM
8.04 is low enough for outside air that I suspect a measurement problem, with the alkalinity or the pH. How did you aerate the sample?

Maives
12/18/2010, 09:53 PM
8.04 is low enough for outside air that I suspect a measurement problem, with the alkalinity or the pH. How did you aerate the sample?

The pH measurement has been verified with borax. I did not know I had to aerate the outside sample, so I will have to repeat that. Alk has been tested with two different API kits. Perhaps not super precise, but should get me in the ballpark.

bertoni
12/19/2010, 12:30 AM
It's best to put an airstone or a small pump in it.