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View Full Version : Questions, questions...ca, alk, pH


JMaxwell
03/31/2008, 03:15 PM
I am confused.

I have a 240 system. Its a 40" tall tank with poor oxygen exchange. The sump holds maybe 30 gal of water.

I have a pretty fair amount of coral in the tank, most of it stony.

I use AquariumWaterTesting.com once a month and presume they are more accurate and reliable than my Salifert tests.

Last year, I started to run a deltec calc reactor on a timer for calcium and alk maintenance instead of manual dosing.

It worked well (CA ~420, Alk ~ 4 meq/l) for a while, but then I decided to buy a pinpoint pH monitor for the tank/sump and was getting readings between 7.6 (night) and 7.8 (day). Too low. Let's start messing with things!

I decided to start adding kalk to my make up reservior. I did it is the simplest fashion because its in a closed cabinet with little air exchange that I only open every two weeks for refill. I dumped a bunch of kalkwasser into the water, stirred it up, and let the dosing pump operate each night.

According to my intramonth manual testing, the Ca and Alk levels started to rise, so I dialed back the time that my Calk reactor ran during the day.

Over this same time period (Jan-Mar) I was getting low Mg readings (1075) so I started adding Mg gradually. As of my last report, it was back up to almost 1300.

Over the last couple of months, my online test reports have shown falling Ca. It went from over 400 in Dec-Jan to 240 in Feb-Mar. Alk fell some in Feb, but has recoved to 4.0 meq/l. (My Salifert Ca kit shows Ca of 500 ppm, but its almost a year old.)

My pH, after a few probe recalibrations that appear successful in the calibration solutions, remains in the cellar at 7.6 at best!

Today, I learned from another of Randy's articles, that the white junk in the bottom of my RO/DI reservior is probably not extra kalk that I dumped in a while ago, but precipitated CaCO3. (Oops.) As a result, I have not been dosing as much kalk as I thought over the past month or so. I'll fix that tonight.

Given the low pH readings, despite what I thought was a mixed regime of Calc reactor and lime water, I decided to try and improve my air exchange. I installed a small airpump outside my house and ran a tube under the tank to an airstone in a glass vase in the sump that receives the calc reactor effluent. I also extended my skimmer air intake hose to the outside of the house.

My questions are these:

Is it even plausible to have a 7.6 pH given an alk of 4.0 meq/l, with the skimmer air intake hose outside and an outside air airstone bubbling in the reactor effluent? Is it possible that my pH probe is junk and I should try a new one? It is only a few months old.

Given that I only dose Ca and Alk in either Calc reactor or Limewater form, how could I end up with Ca so out of balance (low) with Alk? Could my additions of Mg have caused a problem?

Is it possible that I supersaturated something and I should stop Calc and Kalk for a while see what happens?

P.S. I typically run a send bed around 3" deep. A lot of it has dissolved over that last year, so I dumped a fair amount of new sand in early this year. There are any number of creatures in it now and a curious amount of little bubbles that come out of it. Is my sand bed eating up my O2? Am I nuts?

TIA

bertoni
03/31/2008, 03:19 PM
The calcium test results from AWT don't seem to be reliable, in my opinion. I would ignore them, personally.

This article describes an aeration test that would be a good step in diagnosing the source of the low pH problem:

http://www.reefkeeping.com/issues/2004-09/rhf/index.php

Basically, take a water sample and put it outside for a few hours, with a small powerhead or airstone providing circulation. Then measure the sample. :)

JMaxwell
03/31/2008, 03:43 PM
That is unfortunate re: AWT Ca test. Kind of a critical test for such a service.

As for the article, I have it printed out and have been carrying it around with me for months life some kind of life preserver.

Looking at the graphs, is it even plausible to have a pH of 7.6 with an Alk of 4.0 meq/L? (and Ca somewhere in the narrow range of 240 to 500 PPM!)

Could all that activity and bubble formation in my sand bed be eating up my O2 and producing excess CO2, despite my efforts at aeration?

Is there anything else that can cause pH that low? Note that the article really only suggests two reasons: not enough oxygen exchange to remove CO2 or tank is still cycling and producing greater than long run CO2.

My values of 7.6pH and 4.0 meq/l Alk would seem to be literally off those charts, so I don't understand where I'm at.

bertoni
03/31/2008, 04:00 PM
7.6 is possible, but it'd require a lot of carbon dioxide in the water.

I don't know of any data on how much carbon dioxide a sandbed can produce. The bubbling is more likely to be oxygen released by photosynthesis, in my opinion, although it might be nitrogen.

JMaxwell
03/31/2008, 04:29 PM
This may sound silly, but if its oxygen released by photosynthesis, is that adding to or depleting the dissolved oxygen in the water? Would that be a pH lowing phenomenon, and might I watch for readings when that bubbling eventually dies down?

bertoni
03/31/2008, 04:41 PM
Oxygen from photosynthesis can increase the oxygen level in the tank. A change in the oxygen level won't change pH, but photosynthesis also removes carbon dioxide from the water, which will raise pH.

JMaxwell
03/31/2008, 05:36 PM
okay thanks. Action in the sand may be helping my pH