Low pH question, Ca and Carbonate balance

[ereikes]

Hi All,
I have a bit of a tricky problem I've been struggling with for a bit. Let me lay out the system parameters and then I will get to the question :

Tank : 125G
Sump : 30 G
Skimmer ASM G3
Main pump : 600 GPH
Power heads two Koralia 4 (I think 400-500gph)
Refugiem full of chaeto on 100% light cycle
Definitely old tank. Up for several years, moved about 14 months ago with water, LR etc.
180lbs of LR
I have an ATO system that pumps RODI water from a reservoir under control of the RKL. I add Seachem calcium to the topoff water.
I also just started running Seachem Seagel in a reactor to try and combat algae/alleopaths from the corals fighting.

Set up as mixed reef with relatively heavy bio load (fish and eel). Softies, a couple of LPS and 1 new SPS monti. all the corals seem to be doing fine (growing, propogating, etc) with the exception of the monti.

Water parameters :
SG 1.026 (refractometer)
Ca : 430ppm Salifert
Nitrate : 1-2ppm
Nitrite : 0 /salifert
Ammonia : 0 salifert
Alk : 12.2 Dkh salifert
pH : 7.7 at night and 7.9 in the day with DA RKL pH probe
mG : ? don't know, not testing. Ca supplement says it is balanced.
40g water change every two weeks with NSW (Scripps Pier)

Here is my dilemma : Why the heck is my pH so low? I've been dosing with kent marine buffer to try and bring the Alk up (and the pH along with it) clearly I'm at or above the upper range on Alk and still have low pH. I think the kent is bicarbonate and borate salts. Could this be screwing up my test? I just bought some Seachem "buffer". It claims a rise of 0.1ph along with 0.5meq/L with appropriate dose.

My first thought is excess CO2, right? I would think that the G3 skimmer with all those bubbles would blow off any amount of CO2. Plus the power heads, the over flow, the surface area of tank+sump. I did a test where I took out some water and stirred the heck out of it and the pH did not change.

Next thought is the topoff water. I measured its pH right on at 8.3. Checked my NSW also 8.3.

pH has been lowish for a while. Not really trending down, but never coming up to 8.0->8.3.

Weird right?

The next thing I am thinking I will do is just a 110% water change with NSW over the next week or so (broken up into multiple changes to minimize shock). Just to reset any ionic imbalnce and get it back to NSW parameters. Any other ideas? I only live about 2 miles from Scripps so 160G of NSW is really only 4 15 minute trips for me.

[HighlandReefer]

The pH is most effected by the CO2 level in your house around the tank, provided your alk level is within the recommended range.

Your skimmer and surface pumps will mix your tank water with the room air (which is high in CO2) and cause your low pH problem.

Bringing in fresh air from the exterior to you skimmer and using kalk water to supplement will produce the most pH increase.

Randy goes into more details in this article:

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

[bertoni]

Do you mean the SeaChem Marine Buffer? That product is designed for fish-only systems, and I'd avoid adding to a reef tank. The SeaChem Reef supplements are fine, but I agree that kalk is more effective than the pure alkalinity supplements.

I'd try aerating a cup of tank water outside for a few hours and checking the pH. Measurement problems, including bad calibration standards, seem very common.

[ereikes]

Yes,
I have tried the vigorously stirring outside test and I didn't see very much change in pH. I currently have a cup of tank water sitting outside for the day. I'll see if the pH moves around.

There may be some problem with my pH probe, and I've tried to check it out as follows :

If I place it in the 7.00 cal solution it goes very close (.01) to 7.00. Same with the 10.00 cal solution. This means it is at least calibrated to those two solutions.

When I check some recently collected NSW from Scripps pier it is coming in at 7.90. From research on the web most people are claiming this water typically runs between 8.1 and 8.2. So either it has recently changed, or something is off with my cal solutions or probe, or something is off with everyone else's measurement. For now, I am assuming something with my probe setup.

I am going to check the DA website for firmware upgrades and pH problems, etc. I will also track down some cal fluid from a different source and verify my pH test's out.

Why is the seachem buffer for fish only tanks? My understanding is that it is sodium carbonate versus bicarbonate which would have a greater influence on pH for a given change in alkalinity. Its typically not what you want for reef tanks as they usually consume mass quantities of alkalinity, but in the case I have (potential ionic imbalance) it might be helpful getting back into range.

For now, I am in "wait and see" mode. Nothing is dying in my tank (well except my new monti, but that could be for many reasons), I am going to verify my problem before I underrtake drastic fixes.

Thanks.

[Adam_T]

I'm right there with you...all my levels are great but my pH is always around 7.8-7.9. Weird. I might run the airline from my skimmer through the screen in my window to the outside....maybe that will helop! Hahaha.

[Randy Holmes-Farley]

If your meter reads the calibration solutions correctly, and is not suffering from local interference from ballasts and such, then it will read the seawater correctly.

The pH of seawater is determined by the alkalinity and the CO2 level (and nothing else). The seawater collected at Scripps, if allowed to collect extra CO2 in your home, will easily read lower in pH than others might observe who do not expose it to elevated CO2 levels.

[bertoni]

SeaChem has a number of alkalinity supplements. SeaChem Reef Buffer and SeaChem Reef Carbonate are both fine for reef tanks. The SeaChem Marine Buffer is designed for fish-only systems.

If the pH stays fairly low after a full aeration outside for a few hours, I suspect the calibration standards are off. I think that fresher air is likely the issue, though.

[ereikes]

Yeah, I thought about interference or stray currents in my sump, so I also tested the pH in a plastic cup that was outside of the tank completely.

I will share the results of the water I left outside to equalize with outside air when I get home.

As I see it here are the main categories of things going on :

The pH is really 7.9.
1a. It is due to excess CO2 and I should see a difference from outside.
1b. It is due to Alk lower than I think it is. I will test my Salifert alk kit with the included dKH 8.00 solution
The pH is not really 7.9
2a. The brand new cal solutions I bought are off.
2b. There is something woefully screwy with my brand new probe or RKL unit (I am running old firmware as I haven't upgraded).
2c. There is some interference either conducted or radiated screwing up the measurement in the sump.
There is something fundamental I do not understand about pH... well, clearly there is a lot I don't understand, but all of my reading agrees with Highland that it should be CO2 and Alk that determines pH...

I'll keep you all posted on what I find.

One question : How much temperature dependence does pH have? My tank is 79F, while my cal solutions are around 70F. I don't think it should matter that much, but again I only play a chemist on TV....

[bertoni]

The temperature shouldn't be a problem, since the meter is temperature-compensating and the differences otherwise are small.

[ereikes]

Ahh, I bought Seachem "Reef Buffer" sorry if I misspoke.

So, I come home this evening and the pH in my tank is measuring 8.27... just about spot on.

I pulled the tank water I had in a cup outside all day and stuck the probe in there : 7.9... same as I had yesterday.

I just went back and looked and now the RKL is reading 8.04... Almost 0.23 change in pH in about 1 hour with no change in flow, lighting, food or anything.

Methinks the RKL is giving flaky pH readings.....

[bertoni]

I agree that the meter seems to be having issues.

[Reefer08]

Is their anything that will raise pH but not alkalinity/calcium? I love how kalkwasser raises my pH but hate how much it raises my alk/calcium levels.

I really don't wanna redial in my calcium reactor to accomodate for the kalkwasser addition.

[Billybeau1]

Is their anything that will raise pH but not alkalinity/calcium? I love how kalkwasser raises my pH but hate how much it raises my alk/calcium levels.

I really don't wanna redial in my calcium reactor to accomodate for the kalkwasser addition.

Fresher outside air. :D

What is your pH anyways and how do you measure ?

[Randy Holmes-Farley]

Is their anything that will raise pH but not alkalinity

As Billy notes, there is no additive that can raise seawater pH and not alkalinity, although more fresh air (reduced CO2 in the air) can raise pH.