Understanding CO2 / pH / KH

[aaronf]

So I just wanted to make sure that I understand water correctly. I asked this question at my LFS and the response was "to use RO water", which I think, based on what I have read here, is not necessarily correct. I am just wanting to write out what I think to ensure that I have understood water chemistry!

Tank details:

• 90L
• C02 Injection - Added in the back compartment.
• Photoperiod - 6 hours (12pm to 6pm)
• CO2 Period - 7 hours (10 am to 5 pm)
• Fertilizers being done with alternate day liquid macro/mico - awaiting EI kit.

I have been recently changed from RO water to my local tap water (for daily changes):

• WATER SUPPLIER - Total Hardness/kH - 335mg/l (18.75 kH)
• WATER SUPPLIER - Alkalinity - 235 mgHCO3/l
• WATER SUPPLIER - pH - 7.3

I also have a LIMETRON inline, so I measured my tapwater

• TAPWATER - Total Hardness/kH 214.32 (12 kH)
• TAPWATER - pH - 7

I have measured my tank at different points during a 24 hour cycle period.

22:00 (6 hours post photoperiod)

• TANKWATER - kH 8 kH
• TANKWATER - pH 7.0

--- Partial TAP WATER CHANGE --

12:00 (8 Hours post photoperiod)

• TANKWATER - kH - 9
• TANKWATER - ph - 7.0
• TANKWATER - C02 - 20 mg/L

14:00 (2 hours into photoperiod, so "maximum" C02)

• TANKWATER - kH - 10
• TANKWATER - pH - 7
• TANKWATER - C02 - 30 mg/L

TANKWATER DEGASSED - pH 8

Sorry for the information overload! I like measuring

Firstly, I have read that I should aim for a pH drop of 1 during the CO2 phase. With a higher kH buffering this pH change, is that required or even achievable? Based on the degassed pH I am achieving a drop, but it seems like I am not in the middle of the night. I have bought an airstone and was thinking of having this on for maybe 3 hours from 1-4 a.m to add in oxygen and potentially agitate off some C02. Does this sound correct?

Based on the JBL test strips (I am not sure how accurate those are, I backed up the KH measurements with an API test solution however and they correlated). I think I have an adequate c02 bubble rate (I could possibly go a little higher to get to 35). The livestock seems to tolerate this level currently. Would you increase the c02 flow rate? Since increasing plant stocking levels, fert dosing, changing to Tapwater, and upping CO2 based on this forum advice my plant situation is already VASTLY improved even though it has only been 4-5 days.

Finally, is having a high tank kH likely to impact livestock and plants? I have read that SOME livestock can have issues, but not what I currently have.

 

[Gorillastomp]

• WATER SUPPLIER - Total Hardness/kH - 335mg/l (18.75 kH)
• WATER SUPPLIER - Alkalinity - 235 mgHCO3/l
• WATER SUPPLIER - pH - 7.3

total hardness is GH an not KH, alkalinity is KH

Anyways you don't have to care much about the relation between the PH/KH/Co2 relation because many variant in your aquarium can change your PH that is not CO2 related. This why we suggest the drop of 1 PH from your degassed water tank to make sure you have around 30 ppm Co2. So you said that your degassed water is 8, you need to be at least at 7 ph before lights come one.

Something for your should look something like this :

12.00 - Co2 On - PH 8
13.00 - PH 7.5
14.00 - Light on - PH 7
15.00 to 19.00 - PH 7 to 6.9 (small decrease in ph during the photoperiod ensure you have at least your co2 ppm target )
19.00 - CO2 Off - PH like above
20.00 to 12.00 next day - PH ramp up from 7 to 8

repeat.

Since you started to do your WC with your tap water you will have to retest your starting point until you will have mostly tap water in your tank. Your starting point should increase a bit. You will know once your aquarium parameter will look similar to your tap water.

Finally, is having a high tank kH likely to impact livestock and plants? I have read that SOME livestock can have issues, but not what I currently have.

Just add the tap water slowly so the livestock adapt to the new parameter, but i guess you are doing it correctly right now. What would be bad, it would be to pass from your RO water to your TAP water in one sitting, fish would not handle that well since you GH/KH is way different. Most Fish adapt to any water parameter regarding the GH/KH. Discus fish that are supposed to be picky on the water parameter, there is people breeding them in high PH/KH/GH water.

 

[Zeus.]

12.00 - Co2 On - PH 8
13.00 - PH 7.5
14.00 - Light on - PH 7

Gorillastomp is saying 2hrs to get to pH drop, this is a guesstimate, you need to work out what it is for your tank it may be 1hr 45mins or 4hrs it is very tank/setup dependant, once your pH is stable for photoperiod, let the tank degas overnight then time how long it takes to reach your target pH from CO2 on- Do NOT adjust the injection rate as thats the rate needed for stability, the time it takes is the time it takes and whats needed, then set the lights for that time to come on. If you do a WC near CO2 on time you will get a bigger pH drop as the tap water will already have CO2 in it.

 

[ceg4048]

12:00 (8 Hours post photoperiod)

• TANKWATER - kH - 9
• TANKWATER - ph - 7.0
• TANKWATER - C02 - 20 mg/L

14:00 (2 hours into photoperiod, so "maximum" C02)

• TANKWATER - kH - 10
• TANKWATER - pH - 7
• TANKWATER - C02 - 30 mg/L

This is incorrect. You absolutely cannot determine the CO2 content of either the tap water or of the tank water by simply by using the pH/KH/CO2 table. If you do this you will consistently overestimate your CO2. You need to ignore the table and ignore the LFS advice until you have a much better understanding of water chemistry. Have a look at this post=> PH/KH/GH/Co2 problems

Firstly, I have read that I should aim for a pH drop of 1 during the CO2 phase. With a higher kH buffering this pH change, is that required or even achievable? Based on the degassed pH I am achieving a drop, but it seems like I am not in the middle of the night.

This is what happens when testing becomes more important than keeping things simple. You forever chase your tail. Ignore the change in KH because there is nothing you can do about it. You know what the natural pH of your tap water is. Simply subtract 1 and get on with it. Since we lack quality instruments there is no point in attempting to be more accurate that you are actually capable of measuring. The pH drop is a simple guide to get you going, but that is only part of the story. This guarantees nothing and you still need to have good flow/distribution and more importantly you need to observe the tank as it will tell you what's happening regardless of test kit readings.

is having a high tank kH likely to impact livestock and plants?

No. Failing to do large and frequent water changes to control organic pollution, however, will.

Cheers,

 

[Valodia]

Maybe this would help, I like some product descriptions:
"Tetra pH/KH Minus for the controlled reduction of pH and carbonate hardness levels. Precise adjustment of the carbonate hardness and regulating the pH value on a long-term basis. TetraAqua pH/KH Minus is a very safe and simple application. Plant growth is improved due to the release of CO2. Also for the treatment of acute ammonia poisoning."

~£7 GBP per 250 ml , sounds like a bargain for a few grams of acid dissolved in water + plastic bottle.
Also, this probably means that you have to buy "KH" test, pH test, ammonia test, ammonia removal media, pH/KH Plus in case you overdose pH/KH Minus, then some pH Stabilizer to keep it stable, etc,etc.

In reality, if anyone has acute ammonia poisoning in his/her tank, that means biofiltration has failed and/or your water is contaminated with excessive organic waste. The only real cure in this situation - water changes and proper clean up/maintenance.

Regulating pH value short term or long term can not be a goal in general. pH value can be interpreted as an indicator/result of some chemical processes, water composition. For example, achieving ~30ppm dissolved CO2. This means that regardless your water alkalinity (mainly formed of (bi)carbonate ions), whether is it 2dKH or is it 20dkh, 1.0 pH drop approximately means that CO2 concentration went up from 3-5ppm ("degassed", close to the atmospheric equilibrium) to 30ppm (target). In low alkalinity water this drop might be 6.5pH -> 5.5pH, in very alkaline water it can be 8.5->7.5.

This is not 100% accurate, but accurate enough to achieve CO2 concentration around 30ppm+/-10%.
This assumes that:

• there is a sufficient alkalinity buffer and the alkalinity comes from (bi-)carbonate ions, but actually there are much more ions contributing to total alkalinity added by an aquarist as ferts, food etc or coming in your tap water naturally or added artificially by you water supplier. kH tests will actually test total alkalinity, not only carbonate hardness.
• the only acidification comes from a co2 injection only and tiny amount of gas forming carbonic acid, however, there are much more chemical and biological processes producing non-organic/organic acids.

In theory, these two factor should be taken into a consideration, but it is not achievable/feasible at home.