Struggling Conceptually with PH/KH

Jul 21, 2021
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AZ
Hi. I have a 150g tank on a sump. I use C02 injection + Excel (to compensate for sump C02 loss). I have Amazonia ADA on top of Power Sand, then gravel on top of that. I have both plants and fish. I run RODI water with RO topoff. Dosers exchange 8 gallons a day and dose excel and APT throughout the high light parts of the day. I'm using the planted+ 24 hour light. I also have a UV filter. The tank has only been up for 4 weeks, but all of my parameters have been stable. My question is about carbonate and alkalinity and PH. I know that alkalinity is magnesium + carbonate. My alkalinity is always in the low range. My carbonate is also always in the low range. I was expecting the APT to bring it up, but it hasn't. My plants & fish are doing well, but I am concerned my snails, in particular, will be affected by low carbonate. I can add an alk buffer or sodium bicarbonate but I don't want my PH to change too much. Its currently going from around 6.5 to 7.2. 7 is ideally where I want it. Temp is 75. Am I overthinking this? Can I get a boost to carbonate and/or alk without shooting my PH up?

My tank fauna are gouramis, australian rainbows, cherry barbs, mystery snails and hillstream loaches

I appreciate the help. All of this chemistry is fascinating, but oof.
 

naty

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Aug 11, 2021
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Alkalinity is the ability of a solution to resist pH changes when an acid is added. In most natural freshwater systems the main contributors to alkalinity are bicarbonates. Carbonate hardness (KH) is just the concentration of carbonates (CO3) and bicarbonates (HCO3)in the water reported under various units ( °dH, °dKH, ppm CaCO3, mmol/L, meq/L).

Water hardness/ General hardness (GH) is (mainly) the sum of calcium(Ca) and magnesium(Mg) concentrations.

APT is a plant fertilizer and will have very small amounts of Mg, thus it will slightly increase your GH, not KH.
How low is low KH ? What kind of snails? I have ramshorns in 0KH but nerites might not be as happy long term. Adding sodium bicarbonate will increase the KH as you say. The increase in pH will not be very drastic at a moderate increase of KH, say a KH of 4-5°dH.

Regards
Naty
 
Jul 21, 2021
6
1
3
AZ
Alkalinity is the ability of a solution to resist pH changes when an acid is added. In most natural freshwater systems the main contributors to alkalinity are bicarbonates. Carbonate hardness (KH) is just the concentration of carbonates (CO3) and bicarbonates (HCO3)in the water reported under various units ( °dH, °dKH, ppm CaCO3, mmol/L, meq/L).

Water hardness/ General hardness (GH) is (mainly) the sum of calcium(Ca) and magnesium(Mg) concentrations.

APT is a plant fertilizer and will have very small amounts of Mg, thus it will slightly increase your GH, not KH.
How low is low KH ? What kind of snails? I have ramshorns in 0KH but nerites might not be as happy long term. Adding sodium bicarbonate will increase the KH as you say. The increase in pH will not be very drastic at a moderate increase of KH, say a KH of 4-5°dH.

Regards
Naty
Thank you for your reply, Naty. I have mystery snails. I confused Alk & GH. L'sigh. So much to remember. Thank you for clarifying. I am thinking about just adding Equilibrium to bring my minerals up. I think my plants are short on potassium also. My KH & Carbonate are both 40 ppm.
 

Lmuhlen

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Jan 20, 2021
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Brazil
On the topic of Alkalinity and KH, I'm a little confused on how they interact with the CO2 concentrations.

Does the KH test actually measure exclusively the carbonates and bicarbonates, or does it measure Alkalinity as a whole?

On some of Tom Barr's messages, he states that when using the KH x pH x CO2 table, it may underestimate the CO2 concentration, but never overestimate it, and from what I gathered, the reason would be that part of our alkalinity could be due to non-KH substances. In that case, our KH would be lower than our alkalinity and therefore our CO2 concentration would be lower than what the table says.

From those statements, assuming that I got them right, I came to the conclusion that the CO2 and pH relation is bound exclusively by KH substances, not from other alkalinity substances. And also that what we measure is alkalinity and not KH. This last part is what I'm a little confused, because when I tried to reason all this pHxKHxCO2 thing by myself, I came to the opposite conclusion - that the CO2 would possibly be higher than what the table says, because I assumed that the KH test measured only KH, but there would be other alkalinity sources messing with the pH, getting in the way from it reducing when CO2 was added.

Any help with this? I feel like I got myself confused just trying to write the question.
 

Allwissend

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Thank you for your reply, Naty. I have mystery snails. I confused Alk & GH. L'sigh. So much to remember. Thank you for clarifying. I am thinking about just adding Equilibrium to bring my minerals up. I think my plants are short on potassium also. My KH & Carbonate are both 40 ppm.
Equilibrium should add plenty of potassium. The pH sounds decent for snails and if the diet provides good calcium and other minerals they should be good.

Does the KH test actually measure exclusively the carbonates and bicarbonates, or does it measure Alkalinity as a whole?
Far as I can tell all KH hobby tests are actual alkalinity tests. For the wider hobby out there the difference between the 2 would be very small. Like Naty said the main contributors in most tap water would be bicarbonate and carbonate (depending on the pH).

Any help with this? I feel like I got myself confused just trying to write the question.
For reference, here is the original post and chart : https://barrreport.com/threads/co2-ph-kh-table.10717/

What one would typically do is test the KH of the water, test the pH of the water and then go to the chart to see what the estimated CO2 concentration is.
Your thinking is correct, as there may be something else affecting the KH (alkalinity) you would get a value higher than just the concentration of (bi)carbonates. This results in you going down the chart for the same pH and thus an overestimation of the CO2 concentration. Tom Barr says the same thing, here is a quote from here in that thread:
Warning, KH may not be entirely carbonate hardness. This means you will think and believe you have MORE than you actually do, thus you may be under dosing CO2.
This issues will never be the reverse, eg, you are adding more CO2 than you think.

So the error is always on the safe side using this method.
I would add that there are other things lowering the pH value besides carbonic acid (part of the total dissolved CO2). This also leads to an overestimation. For example, an aquarium with a KH of 6 may have a pH of 7 without adding CO2, just from weak organic acids such as tannins. That doesn't mean the CO2 concentration is anywhere near 18ppm CO2. The aquarium will still be ~at equilibrium with the atmosphere. I've had many tanks where the value of CO2 from the chart would be in the hundreds of ppm CO2 but the fish were fine and the indicator only a pale green. I suspect the lowering of pH from other sources (in that case aquasoil) was more to blame than a deviation from the true KH value.

The point is that the chart may not be reliable in an aquarium env. I would suggest the "pH drop method" to estimate the concentration of CO2. This works better in the medium range of bicarbonate concentrations. The "ph drop method" aims for 1 unit drop in the pH value between "degassed" water and the target.

"Degassed water" is water at equilibrium with the atmosphere. Take a small amount of aquarium water in a clean glass and allow it to reach equilibrium with the atmosphere in a well ventilated room. Measure the pH. This will be the pH of water having about 3ppm CO2. Then adjust the CO2 injection so that the pH of the aquarium water is 1 pH unit lower than the one for the "degassed" water. This will be about 30ppm for most medium KH values. You can follow it on the chart. This method aims to reduce the impact of other things on the aquarium's water pH. It is however still an estimation.

For example, aquarium water with a KH of 5 has a pH of 7 at 6am ( from accumulated CO2 and other stuff). I leave it to 'degas' and reach a pH of 7.4 ( from ~3ppm CO2 and other stuff). I then adjust the CO2 of my tank so that I reach a pH of 6.4 (7.4 - 1 = 6.4) around the time the lights turn on to have around 30ppm CO2.

Please as always, when you increase the CO2 do it gradually and monitor the fish condition over the next few days.