Controlling PH with CO2 question

[nycman]

I have a large tank 200G with overflow and sump. After several tweaks on the plumbing, I have virtually no surface agitation and ability to overdose on CO2 (based on my history with this tank, a good thing). I am getting Discus and want to be more careful with the daily PH swings. I run the CO2 8 hours a day inside of a 10 hour lighting period. Without the PH controller, my PH would peak at 7 (my tap water PH) just before the CO2 would come back on, and then during the 8 hour CO2 cycle dip to upper 5 or lower 6, depending on my bubble count tweaks.

Here is my question on setting up my controller...with Discus (or any sensitive fish), should I:

1) run my CO2 24 hours a day with the controller auto shutting it on and targeting a specific PH - I have tested it and can keep it very stable at 6.3 to 6.5, and still get ample CO2 into the tank. This way the PH of the tank will be at this value 24x7 as I will be running the CO2 at night as needed for this PH.

2) run it on the 10 hour lighting cycle, shutting it off at night and allowing the PH to creep back up to 6.8 or 7. This way the daily PH fluctuation will be 6.3 or 6.5 to 6.8 or 7, not insignificant, but not huge.

Assuming both approaches are equally appropriate for the plants - which is better for the Discus?

 

[tanker]

IMO, using CO2 to control PH is a disaster waiting to happen. I would rather use RO water and adjust accordingly.
Do you have plants?

 

[nycman]

Yes, the whole point of my tank is to have plants with CO2. The fish are secondary. I have achieved CO2 perfection for the plants, after much tweaking. Using Rex Grigg reactors (2x) with electronically controlled regulators (2x). All on Apex. Now I want to add Discus, and want to minimize the PH fluctuations (consistent daily) due to CO2 injection.

I am using tap water. I started with RO/DI (as I have a large reef tank and a major RO/DI system), but it was terrible for the fluctuations due to the low Kh. My PH would swing from 6.9 to 5.2 everyday. I visited my friends at Pacific Aquaria on Delancy Street in Manhattan and they advised tap water to get minerals, phosphates, higher PH, and improved Kh and Gh. I know buffers and additives can fix these deficiencies, but why make it so experimental and complex if tap water is stable. My tap has low TDS and a PH of 7.0.

To be clear, during the 8 hours I am injecting CO2, the PH drops from ~6.9 to ~6.2 on a highly consistent daily cycle. This fluctuation is what I am trying to solve for.

 

[OrionGirl]

Does the hardness fluctuate during the pH shifts?

 

[SnakeIce]

The main point is that you keep on top of total dissolved solids (tds) because that is the main source of "ph" stress when both change to much. A ph change caused by CO2 does not change tds significantly and as long as there is sufficient Oxygen also dissolved there is not much effect on fish assuming the CO2 is not wildly over the 30ppm target. There are other issues that arise long before "ph" tds shock sets in due to CO2 injection.

There is an issue with starting ph creep that effects how much CO2 you have dissolved if you are using a ph target to control the amount of CO2 you are injecting. One log ph difference is 30 ppm CO2, so if your tank starts out at 6.9, 5.9 would be what you see when 30ppm CO2 is achieved. So you set your controllers to hit that target 5.9 ph, which is all good and well, if the starting ph of the tank remains 6.9.

As long as you are doing enough to replace the buffer used up by metabolic processes that won't be a problem. You can check the effective ph change you are getting from CO2 by setting a sample of tank water in an open container overnight and testing the ph of that sample once it has reached ambient CO2 levels. The difference in the ph of the tank to the sample is directly due to CO2 injection. That simplifies accounting for the organic acids and bases present in tank water in addition to the mineral ph of the source.

If you have the ability to turn the CO2 off at night, why not? Getting cylinder refills is not a insignificant cost and by not running the system during the time the plants are not using CO2 you can stretch the length of time a refill lasts. Oxygen production by plants is in negative numbers at night also, so the risk of CO2 stress on your fish is higher at night also. Assuming the plants are growing well the production of Oxygen during the day should be enough to over saturate the water which is more than enough to last the night, but if there is any issue it would be slightly safer to not run CO2 at night.

One method I've seen used is to turn the CO2 on 30 minutes before lights on, and off 30 minutes before lights off.

 

[nycman]

The main point is that you keep on top of total dissolved solids (tds) because that is the main source of "ph" stress when both change to much. A ph change caused by CO2 does not change tds significantly and as long as there is sufficient Oxygen also dissolved there is not much effect on fish assuming the CO2 is not wildly over the 30ppm target. There are other issues that arise long before "ph" tds shock sets in due to CO2 injection.

There is an issue with starting ph creep that effects how much CO2 you have dissolved if you are using a ph target to control the amount of CO2 you are injecting. One log ph difference is 30 ppm CO2, so if your tank starts out at 6.9, 5.9 would be what you see when 30ppm CO2 is achieved. So you set your controllers to hit that target 5.9 ph, which is all good and well, if the starting ph of the tank remains 6.9.

As long as you are doing enough to replace the buffer used up by metabolic processes that won't be a problem. You can check the effective ph change you are getting from CO2 by setting a sample of tank water in an open container overnight and testing the ph of that sample once it has reached ambient CO2 levels. The difference in the ph of the tank to the sample is directly due to CO2 injection. That simplifies accounting for the organic acids and bases present in tank water in addition to the mineral ph of the source.

If you have the ability to turn the CO2 off at night, why not? Getting cylinder refills is not a insignificant cost and by not running the system during the time the plants are not using CO2 you can stretch the length of time a refill lasts. Oxygen production by plants is in negative numbers at night also, so the risk of CO2 stress on your fish is higher at night also. Assuming the plants are growing well the production of Oxygen during the day should be enough to over saturate the water which is more than enough to last the night, but if there is any issue it would be slightly safer to not run CO2 at night.

One method I've seen used is to turn the CO2 on 30 minutes before lights on, and off 30 minutes before lights off.

Brilliant reply...four stars. Thanks much. I will do this...shut off CO2 at night with a 30 minute shift forward, and then use the PH controller to shut it off (and on again) as it bounces on and off the low range (where max CO2 ppm is occurring. Thanks.

 

[SnakeIce]

The only caution I see for such a setup is if one of the sensors for this were ever to not be submerged in your tank it could turn the CO2 on full blast. If you also have a way to set the bubble rate to just a bit over what is needed that would be a second line of defense against a livestock loss. That way if for whatever reason the ph sensor fails or is not in position to work correctly the full on CO2 rate is not at nuke the tank levels. The plants would be fine, but the animals wouldn't do so well with maxed out CO2.