Aeration for true pH (gassing off CO2)

[DSenn]

One of the more useful tidbits I've learned on here is that the pH is lower out of the tap because of the CO2 trapped in the water, and that you have to let it sit out or aerate it to get the 'true' pH. Therein lies my question: Is my 'true pH' (for lack of a better phrase) the water straight from the tap without sitting out or aerating, the water that simply sits out for a while, or the water that has been aerated?

Recently, I have been taking my tap water and putting it into a bucket with an airline tube and air pump. This drives the pH from around 7.2 up to 7.8-8. Which of these is the pH I should follow? I feel like I am influencing the true pH by adding too much oxygen, but at least this way I know how high my pH COULD go. Maybe just letting the water sit out overnight is the more accurate way to get a true equilibrium? I'm not sure.

 

[bs6749]

The "true pH" of the water that you need to worry about is the pH when your water comes into equilibrium. I'd use the 7.8-8.0 as the "true pH" but I wouldn't be worried about adjusting it unless you are trying to breed certain fish or are trying to keep delicate species such as some caridina species shrimp that need a soft and acidic water to survive/breed.

 

[Purrbox]

Your true pH is the pH reading after allowing the excess CO2 to gas off. This can be achieved by aerating your water for about an hour or leaving it sit for 24 hours. You should get the same results from either. Oxygen does not affect the pH reading so you needn't worry about aeration mucking with the results. If the 7.8-8 pH reading is after at least an hour of aeration, that would be your true pH of the tap water.

 

[DSenn]

That's pretty much what I figured. I just thought that I was possibly 'forcing' a slightly higher pH by aerating it too much (therefore manipulating the equilibrium it would reach naturally). Like I said, I aerate my tap in a bucket to see how high the pH can go. It usually ends up in that 7.8 range. Then, I mix 1.5 gallons of this tap to 3 gallons of R.O. in a 5 gallon jug. This pH ends up around 7, and will stay at 7 when I let it sit. BUT, if I put the airline tube in this jug, the pH spikes up again. Not quite as high as 7.8 (closer to 7.4), but it seems like that extra aeration is messing up the equilibrium it would find by just sitting out. I am interested in this issue because every time I put my water in my tank, my pH tends to rise (I assume from the aeration of my biowheel) and I am trying to avoid the high pH.

 

[jsoong]

... Then, I mix 1.5 gallons of this tap to 3 gallons of R.O. in a 5 gallon jug. This pH ends up around 7, and will stay at 7 when I let it sit. BUT, if I put the airline tube in this jug, the pH spikes up again. Not quite as high as 7.8 (closer to 7.4), but it seems like that extra aeration is messing up the equilibrium it would find by just sitting out.

I think I figured out what is going on here!

You have not aerate your R/O water after the filter, so it is still full of CO2. hence after mixing the pH rises again.

This is the theoretical values I calculated:

Tap: KH 270 (ppm), pH 7.2, CO2 30 (ppm)
Tap after aeration: KH 270 (no change), pH 8.0, CO2 4 (atmospheric)
R/O (no aeration): KH 20 (1 degree you measured), pH 6.0, CO2 30 (no aeration)
R/O after aeration: KH 20, pH 7.0, CO2 4 <what R/O should read>
mixing unaerated R/O with aerate tap: KH 100, pH 7.0, CO2 20
mixing aerated R/O with aerated tap (= your mix after sitting out): KH 100, pH 7.5, CO2 4

As you can see, your tap water is acting exactly (close enough for me!) as predicted.

Anyway, the "true pH" you need to use is the water after aeration. O2 has little effect on pH, so you don't need to worry about over aerating the water for a reading.

 

[DSenn]

Some of your numbers are off jsoong, some are right. My tap KH is not that high. It is only about 7 degrees tops. The pH out of the tap is about 7.2. After aerating it rises to 7.8-8.

I am glad you mentioned the aerating of the R.O. water, because I had never done that. But, my R.O. pH is below 6 and there is no way my R.O. has a KH of 1. I was referring to the KH of 1 in terms of the mixture of R.O. and tap. My current ratio of 3 gallons R.O./1-1.5 gallons tap gives me a KH of 1, maybe 2 degrees. However, I am now aerating both my tap and my R.O. together in the same jug.

My mixture of aerated R.O. and tap (3:1) is 1-2dKH, pH 7-7.2, CO2 unknown

The GOOD thing now is that the parameters of the water I am 'making' is staying constant after I have added it to my tank. pH of 'made' water, 7-7.2; pH of this water after added to tank, same. Before, the water I was making wasn't aerated and was 7-7.2. After added to tank, it was 7.8.

 

[DSenn]

I also have one more question that is super stupid, but I just want to make sure. My GH/KH test kit is add one drop, shake, check color, then add another drop, shake, check color, etc. So, when I test for GH/KH, I usually get a drop of blue (which I assume is 1dKH). Then, I add another drop, and the liquid will go from blue to yellow. My assumption is that I have between 1 and 2 dKH. Sound correct?

 

[DSenn]

Now I'm testing the water in my tank and it is back up to a 7.6 again. I give up.

 

[jsoong]

My calculations are just ball park estimated! However, I remembered in your other post that your KH out of the tap was 16 or so, so I used that as a starting point.

Anyway, one thing I need to know is if there is anything in the tank that may be changing your KH .... such as limestone, crush coral, sand, etc. For now, I think you need to see how your water behave in an empty container to cut down on the variables.

Assuming that you have a pure carbonate buffering system:
If your tap after aerating has a pH of 7.8-8, then your KH should be 150-200 ppm.
Assuming that your R/O unit is 100% efficient, (ie KH=0 in R/O water), then mixing 3:1 should give your a KH of 40-50 in the mixed water.
This mix should theoretically have a stable pH of 7.3 at atmospheric CO2.

I am unclear on how long you let your mix aerate & sit before declaring it stable. Maybe you should measure the KH in the tank & see if it had changed from that of the mix.

My thoughts ATM:
1. Something is leeching HCO3 into your tank water - in that case your KH should be rising in the tank.
2. Since our KH kit has an error of +/- 20 ppm, and the pH kit +/- 0.2, all the measurements so far are still within error limits when compared with theoretical. In that case, what you are observing is incomplete aeration of your mix, and the pH is rising as it equilibrate completely in your tank. <If you aerate in a milk jug, for example, there might not be enough air exchange between the air inside the jug with atmosphere.>

At any rate, if the tank pH is stable, you might just accept that & not tear your hair out!

 

[DSenn]

jsoong, check this out: Here are my numbers again, recently tested...

Tap Water:
pH straight from tap, unaerated: 7.2ish, give or take
KH: 7-8 degrees (7-8 drops with my test kit), which I guess is around 130 ppm or so
pH aerated: 7.8, 8ish. You already know this.

Now, THIS is what I've been doing:
I batch up 3 gallons R.O./D.I. water and mix this with approx. 1 gallon of tap (3:1 ratio). Then I aerate the two TOGETHER (all 4 gallons) in a 5 gallon jug for at least an hour or 2, sometimes more, or until I see the pH as not rising that much higher.

This mix gives me a pH of AROUND 7.0 (sometimes a little higher, sometimes a little lower. I am shooting for a flat 7 at this point (simply because of low KH) so if it is a little higher, then I add back a TINY amount of R.O., a vice versa if the pH is lower than 7).

With this mix, my KH drops down to 1 degree, maybe a little more, maybe a little less. The test kits are funny for a very low KH in that they don't 'light up' as well as if there is a higher KH present.

So, aerated mixture of R.O./Tap 3:1 = pH 7ish, KH 1ish.

So assuming my water is carbonate buffered, I am led to believe that I cannot achieve a lower pH than 7 without going lower on my KH (or introducing CO2 into my water). Correct? And I am also led to believe that, if I wanted to do a soft water plant tank, I should probably aim for water with a KH of 2-4 degrees, and THEN lower my pH to around a 6.8 with CO2? Is this was people do to get their pH down in the 6's?

I also have a couple more questions...If my pH were to be unstable, what kind of 'swings' would I encounter? For example, if I tested my water once every five minutes, and if my pH was swinging, what kind of differences would I see? A 7 to a 6?

Also, have you ever done a soft water tank? If so, what were your own parameters?
Finally, might I add, that I have NO fish in my tank currently. I would assume that after they are in there for a bit and my nitrates get going, the pH will drop on its own.

 

[DSenn]

bs6749,

You seem to keep a few soft water tanks. What is your tap water like? Do you use R.O.?

 

[jsoong]

OK, I checked your latest numbers & it seems your water is mostly carbonate buffered.

The way the pH changed, both with aeration & with the mixing of R/O water are all within the range of my calculations. So far, so good. But why is that water's pH rising in your tank? Most likely some carbonate source. Did you check the KH after the water had circulated in your tank?

If you use a carbonate buffer, at a KH of 1, you would have pH around 7.0 after equilibration with atmospheric CO2. Adding CO2 to lower the pH does not make water soft. Soft water (from the fish's standpoint) refers to the amount of dissolved solids, CO2 doesn't change that. Although pH usually increases with hardness, this is not necessary true. <Thus your water right out of the tap, even though the pH is 7 due to the CO2, is still considered moderately hard.>

Your pH swing depends on how much buffer you have. If you have no buffer whatsoever, just equilibrating the water to atmospheric CO2 would drop the pH from 7.0 to 6.2. Having just 1 degree KH buffers the change to around 0.1 units. <That's what buffers are used for, to dampen the pH swing.> So if you have 1 or 2 dKH, it is quite unlikely to see much pH change from accumulation of nitrates. <This is a GOOD thing!>

 

[DSenn]

jsoong...

Aeration is why the pH went up BEFORE. The last time I had my tank filled (which started this discussion) I was using 3-4 gallons of TAP and 1-2 gallons of R.O. This left me with a pH of 7-7.2 UNAERATED. The pH in the tank remained here before I turned the filters on, and the next day after the filters were turned on, the pH was up to 7.8.

I have emptied my tank out and started over, only THIS time I am using 1 gallon tap and three gallons R.O., AND I am aerating with an airline tube BEFORE putting it in my tank. My ph is around a 7 and my KH is around a 1. The previous time, my KH was a 4. Now, if my pH decides to rise on me, it can't possibly go up that much higher. There is barely any KH, and most of the CO2 has been aerated off.

What I have in my tank for decor are clay pots, and a lot of them in crazy arrangements. You'd have to see a picture to truly understand what I mean, but that is for another post. Anyway, you were wondering what I had in there that could be adding to my KH. The answer is nothing. Clay pots are inert. I have even sat them in a bucket with R.O. water, and I recorded no change whatsoever. Unless something freakish is happening with these pots adding some sort of alien KH, etc, then the pH spikes in my tank simply because it is (like you said) coming to equilibrium with the atmosphere.

Finally, you said that 1-2 dKH will help prevent against changes in pH from nitrates. I am assuming that the more partial water changes the better for me with such low KH, because it will add KH back while removing nitrates simultaneously?

Thanks for the help!

I'll post again when I have my tank all the way full and I turn the filters on to see where my pH is.

Oh, and one more question...CO2 lowers pH, but what effect does it have on the KH itself? CO2 forms carbonic acid, so does the KH get used up when CO2 is added? If the CO2 lowers the KH, I assume it would be a good idea to have a few degrees KH? Whew.

 

[Carissa]

Hi,
My tap water has 0 KH and GH, so I'm very familiar with using soft water.

Adding co2 does absolutely nothing other than lower pH by the addition of carbonic acid to the water. This type of change really doesn't affect the fish much. I injected co2 in the past, and have changed pH in my tank by as much as 1.0 pH in 24 hours, with no apparent effect on the fish. Incidentally, KH has no effect on the amount of change in pH that injecting co2 will have. I'm just telling you this, since there is a lot of misinformation out there where people claim that you need to buffer your tank before you use co2 to prevent pH swings. It doesn't work that way. But at the same time, it doesn't matter to the fish, so why mess around with buffers (unless there's some other reason).

Generally speaking, pH doesn't really matter a roll of beans. It's the KH, GH, and TDS that matter to fish. Plants need a certain amount of GH, at least about 4 degrees if possible (they use Ca and Mg which is what GH is). Besides that, pH/KH doesn't matter much. You can grow nice plants in hard and soft water. There may be some very specific types of plants that require one or the other, but by and large, pH doesn't matter. Fish care more about a stable KH/GH than they do that it's at a particular level, unless you are breeding or keeping very sensitive fish.

So it's interesting to test pH and see where things are at and learn about what's going on, but don't fuss over it. It's better to focus on the reasons WHY pH is off - is it low/high KH, a lack of water changes (causing waste buildup which causes acid conditions), etc. These are the things that will actually affect the health of your tank, not the pH test. And once you start injecting co2, you might as well give up on getting any meaningful information from your pH test kit, since it will fluctuate wildly depending on how much co2 you have injected at any given time.

 

[jsoong]

Dsenn - I was confused! On the order of the posts, I thought that you had mixed up your new R/O Tap mix, got it stable, then had a pH change in your tank. But it seemed you are talking about your last batch, and this new batch is stable??

As most people had been saying all along, it is not a perfect pH that you should be chasing. Rather, stability is the key. Your fish & plants will do fine a bit out of ideal range, as long as it is stable.

 

[DSenn]

jsoong-

My last batch: 3-4 gallons tap/1-2 gallons R.O., with a KH of 3-4, pH of 7.8 AFTER aeration IN TANK. I was not satisfied with that high of a pH, so I tried some pH Down (considering I had lowered the KH to 3-4, I thought it would work well) Well, the pH came back to a 7.8.

Current Batch: 1 gallon tap/3 gallons R.O., with a KH of 1, pH of 7.0 after aeration IN JUG. I have a 75 gallon, so it has taken me a bit to get it full, and my filters have NOT been turned on yet. Like I said on my last post, I don't see how my pH can go any higher considering it has already been aerated and my KH is a 1.

In terms of stability, that is what I am trying to figure out. I have no fish in my tank yet, obviously. I am trying to get these parameters down before I add fish.

Also, I understand that if my parameters are off that my fish can adjust. That really isn't the point behind this. I am trying for a SOFT WATER TANK. Since fish have the ability to adapt, then my fish will adapt to soft water. They will appreciate it in the long run.

jsoong, your last post gave me a little piece of mind. It sounds like my current numbers (KH1, GH2, pH 7 ish should be pretty stable. My plan is to keep it here, add fish, and do lots of partial water changes to keep the hardness fresh.

 

[jsoong]

Carissa - I agree with most of what you said. It is true that fish tolerate pH drops from CO2 far better than addition of acids. <Likely because CO2 drops the pH without altering osmolarity, while other acids uses up KH, and the resultant pH crash is associated with a drop in osmotic pressure.>

But for any target CO2 level, having a few degree of KH decreases the magnitude of the pH change by a large amount, so why subject your fish to the pH swing if it is not necessary? Also having some buffer guards against a pH crash from organic acid accumulation (from the fish's metabolism), it is a good insurance against your missed water change, accidental overfeeding, etc.

 

[jsoong]

DSenn - I am glad we sort out which batch of water we are talking about or there will be no end to the confusion!

Your current mix should be reasonably stable as long as you keep up with water changes and not let the KH deplete. <This is for when you add fish.>

This current thread:
http://www.aquariumadvice.com/forums/f60/reverse-osmosis-106802.html
has a good take on R/O and the need for adding buffer. The poster's experience with discus & pH stability is that he end up shooting for a TDS of 100 to keep pH stable. This would be in the range of 2 degree KH, so you might consider setting your KH target just a bit higher.

 

[DSenn]

jsoong-

Yeah, I was actually reading that thread recently (among many other sources). If I upped my KH to 2 degrees, then what do you think my pH would end up at, considering what you know of my water...a 7.3? 7.4?

Also, when I am checking for my KH, I add 1 drop from my test kit, and it turns a light shade of blue. I add another, and it turns yellow. Is it safe to assume that my KH is BETWEEN 1 and 2 degrees?

And FINALLY, TDS stands for total dissolved solids I assume, so is that a combo of the GH and KH? If I had 2 dKH and 2 dGH, would that be around 75-80 TDS?

And FINALLY FINALLY, I just want to say this. This whole KH thing is a royal pain in my a$$. I lower it to get a lower pH, and then when I lower it too much I am in danger of pH crashes. I'm like, what do I do? I know if I raise my KH even the slightest amount, my pH is going to jump back into the mid 7's. If I keep it where it is at a 1, then it is very low. GRRRRRRRRR.

 

[Purrbox]

TDS does stand for Total Dissolved Solids, but it is all solids organic and inorganic. You need a TDS meter to measure it.