CO2 w/ wet/dry filter

[Biotoper]

I've been running DIY CO2 on my 75gal and not seeing any effect on ppm. I've got 4x2L bottles, hooked up with individual inverted IV tubes to count bubbles, so I know they're bubbling at good rates. I originally had it connected to an airline going into the base of my wet/dry filter, but I wasn't seeing a drop in pH. In fact I was seeing swings of ~0.4 pH from morning to evening (7.8 in the morn, 8.2 in the evening) - KH stayed constant at 4.5, so we're talking 1-2 ppm CO2 roughly.

Yesterday I built a DIY reactor (powerhead, vacuum tube, airstone), hoping for some improvement. There're bubbles bouncing around in the vacuum tube (none really escape out the ~8" tube), but when I measured params this evening, pH 8.4 and dKH 3.5! So CO2 actually went down. Now, it is at the end of the day when I'd expect CO2 to be lowest, but come on, man!

Now, nitrates are 0, and all but my wisteria are growing pretty well. So maybe they're pulling out all the CO2 I'm adding? Or maybe it'll take a while for my DIY to saturate all 75gal? Or maybe, all my CO2 is gassing off the trickle filter, and all this is an effort in futility?

I should also note that my tap water is strange in being very soft and basic - pH 8.4 and dKH 2.5. I originally added baking soda to raise the dkH to ~4.5, but have stopped, hoping a couple pieces of coral & tufa rock I have in the tank would keep the dKH stable. I do 5% water changes every 4 days or so. Oh, and I have 160W NO - because of the filter compartment in the back, it's only about 60g that is lit, so I'm somewhere in the 2-3 w/g range.

 

[hashbaz]

I'm guessing that that you are not getting enough CO2. DIY is probably too small for a 75 (or even a 60) even with 4 reactors. Another thing to note is that your wet dry filter will also lower your CO2 levels.

Your tap water parameters (pH 8.4 and dKH 2.5) tell me you have another dilemma on your hands. This gives .3ppm CO2. I doubt the level could be that low. The CO2 charts are only valid when CO2 and carbonates are the only thing affecting the PH. I know that high phosphate levels can raise your ph and throw off your CO2 calculations - but I don't know for sure what else can.

This is one subject that I really wish to understand better but haven't taken the time to research. I have found a decent site, but I still have questions.

http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm

edit: I just read some more on that site and you probably do not have a problem with phosphates like I was thinking. I'm guessing that the ph of your tap water goes down after sitting for a while, as more CO2 is absorbed.

 

[malkore]

Whenever I see water like yours, I get confused, because it really breaks the 'rules' of the pH/Kh relationship as I understand them (and bear in mind, my degree is in English, not Chemistry).

With a dKh of 2.5 your pH should barely be above 7.0. See, when water goes up in pH, it's alkaline, rather than acidic. However you have very little alkalinity (carbonate hardness)...so your pH shouldn't be anywhere near that high.

Are you on city tap water? Any water softner in the house? Have you had your water tested by the LFS to ensure your test kits aren't bad?

 

[Biotoper]

Thanks for the input. I'm on the Boston, MA MWRA city water. The monthly report says the closest testing point to me is pH 9.3 and dKH 2.2, so my test kits seem in the right ballpark. Total phosphorous is 0.016 ppm, TDS is 105 ppm, dGH is 0.8 (I don't have a GH test kit). After pumping through my tank, the pH drops to 7.8 (w/ or w/o the DIY CO2), so it's taking up a little CO2 from the air - adding the baking soda didn't have a measurable effect on pH. I believe this is because the town adds something, maybe carbon hydroxide?, to reduce leaching of lead from old pipes, which raises pH by 2.5 but only increases dKH by 2.1 (the reservoir is pH 6.8 dKH 0.15!)

I tested the pH this morning - 7.8. This daily pH swing has probably been going on the whole time I've had my fish (I planted it before adding fish). I've had 3 fish deaths - 1 rasbora got fungus which I assumed was secondary infection from a cut he got when he jumped into the overflow before I put a wall above the tines (real struggle to fish him out) and 2 loaches - 1 looked like he just wasn't eating and I found dead, the other just disappeared!? I read that small clowns (I got them about 1") have a high mortality and it's better to get them bigger, but maybe my pH swings were the cause The other fish seem very happy....

So what do people recommend - I 'm up for adding more bottles, but I'm hesistant to buy a pressurized CO2 setup if so much is going to be wasted due to my wet/dry. Maybe I should just be happy with the moderate plant growth I have now and forget CO2.

 

[jsoong]

You town has added something to the water to raise the pH to prevent leaching, based on your pH/KH numbers. <Our city water co also do the same if the pH is less than 7.6 or so - but our HCO3 is usu. pretty high, so this isn't necessary except during spring runoff.> Whatever it is they added, it is a secondary buffer, and will completely invalidate your pH/KH/CO2 tables.

The actual equation used to estimate CO2 in those tables actually uses pH, CO2 & [HCO3-], and they assume that KH measures [HCO3-]. With a secondary buffer in your water, your KH test no longer measures [HCO3-], no wonder you are getting strange numbers from your tests.

In most aquarium, esp. with a wet/dry, you water should be equilibrated to room air CO2 <that is without CO2 added>. If you do a KH/pH test & find your CO2 less than room air (=3 ppm), you have something wrong with the test! In your case, the secondary buffer is messing with the KH=HCO3 asssumption, that's why you are measuring 1.5 ppm of CO2.

My chemisrty isn't good enough to do mixed buffer calculations .... in my work (dealing with people), HCO3 predominates & we ignore all other buffers in our calculations ... plus I have machines that measures CO2.... so I don't know what to make of your CO2 levels. Depending on what buffers are present, your CO2 can be sky high ... you are just not measuring it! This may also contribute to fish death.

From my reading regarding CO2 & aquarium, injecting CO2 is not recommended when secondary bufferes are present as you cannot accurately measure CO2 levels .... although one guy says you can use the DIY system in a big tank as it is not big enough to get CO2 up to lethal levels ... but definitely NOT pressurized unless you can measure the CO2 directedly ... <Work in a lab or a hospital? talk to a respiratory teck to see if they will run your water through the blood gas machine...>

 

[Biotoper]

Man, it's like having a light bulb over my head! Thanks, jsoong, for finally figuring out what's going on. So my KH test kit (and the alkalinity reading on the monthly report) is actually underestimating HCO3, and thus I likely have much higher CO2 concentrations? But I'm still getting pH swings, so what's that mean? Now, the fact that pH doesn't change w/ or w/o my DIY CO2, doesn't that mean that I'm not really injecting much, independent of the fact that I'm not measuring HCO3 accurately? I'm sure with all the aquarists in Boston, someone must have a lot of knowledge about dealing with secondary buffers, but this is really the first time I've read about this issue. I've gotta join a local aquarium club.

My sister is a nurse (she got me the IV tubes), so I'll ask her about a blood gas machine. She'll laugh at me. Thanks.

 

[jsoong]

Actually with a 2nd buffer, you are overestimating the HCO3 with the KH test. The actual CO2 level however, can't be predicted (by me at least!) as the H+ level is now dependent on 2 buffers.

This might make it clearer:

CO2 + H2O -> H+ + HCO3-

This is the basic equation in a carbonate buffered system. You measure the H+ (pH) & HCO3 & estimate the CO2 based on the dissociation of the H2CO3 molecule.

Once you added a 2nd buffer, the H+ level is also dependent on the other buffer. In my city, they add Ca(OH)2.

Ca(OH)2 + 2H+ -> Ca++ + H2O

So the second buffer "locks" up the H+ (in this case, it becomes water so you can't even measure it! ... although the Ca++ will now show up in a GH test).

So you can see ... in extreme case .... you can have lots of CO2 in the system, but the pH won't change (much) as all the H+'s are mopped up by your 2nd buffer. <The actual pH change will actually depend on how much & what the buffers are & their respective association constants ... as I said before, I am not good enough to handle the math! >

 

[Biotoper]

Oh, well, I'm totally confused. So I'm overestimating KH because I have other anions (presumably hydroxide) contributing to the alkalinity. So the true carbonate hardness is even lower. That would mean my CO2 is even lower than what I was calculating off my KH reading. But you're also saying that maybe my pH reading is not correct?! - because cations are decreasing H+? I really need to study up on my chemistry - I used to be good at this stuff
Looking at the monthly report again, post-treatment Calcium is about 2x and Sodium is about 6x what they are in the reservoir. So does this mean my water is actually acidic, and my tap water is chock full of CO2?!

 

[jsoong]

So the true carbonate hardness is even lower. That would mean my CO2 is even lower than what I was calculating off my KH reading. .... So does this mean my water is actually acidic, and my tap water is chock full of CO2?!

No ..... your pH reading is correct.... Your water IS alkaline - It's just that the alkalinity is not caused by carbonates.

In a non-carbonate system ... you cannot use the tables you have to caluculate CO2, as you are trying to do. Those tables are generated assuming only HCO3 in the water & nothing else. The 2nd part of the last post was supposed to show why this is so.... <sorry if this is causing confusion .... I should have simply said - DON'T use your KH/pH/CO2 tables>

That 2nd part is an example of how a second buffer CAN remove the H+ generated by the CO2, and so can in theory have a system which is chocked full of CO2 & still have high pH. This is just an example ... I am not saying that this is so in your tank! As I said ... I can not calculate the CO2 in your tank .... although I don't expect it to be too high with your setup.

 

[Steve Hampton]

Actually there is a way to estimate CO2 levels when your buffer is not mainly carbonate/bicarbonate. Take out a bowl of water from your tank and let it set over night. Take a pH reading of the water in the bowl after it's been out over night. Note that this water should have roughly 2-3 ppm of CO2 at equilibrium with the room. From that pH, meaning the pH of the water in the bowl you have a known (reasonalbly known) starting point. Let's say for the sake of this example your bowl of water has a pH of 8.0. When CO2 dissolves into water an exact amount forms carbonic acid, we depend on this known amount for the formulas that create the CO2 charts. Anyway, what this allows us to know is that regardless of the pH starting point if you dissolve enough CO2 in water to form enough carbonic acid to drop the pH 0.6 to 1.0 you will have roughly 15 - 30 ppm of CO2...meaning if the starting pH from the bowl of water is 8.0 then you need to add enough CO2 to drop the pH to 7.0 - 7.4. Note that you must ignore the KH reading and CO2 charts. Note also that the reference pH can't be your tap water or your tank water but it must be from tank water in a bowl left out overnight..

This is not without some hazards. If you have calcium carbonate rocks or decorations in your tank they will begin to slowly dissolve with high levels of carbonic acid in the water...this can result in a slow increase in the amount of bicarbonate and overdosing of CO2. Just like you should do with a tank that has a single buffer you need to regularly test the pH...it's just that your pH needs to be done outside the tank to continually check the reference point. Hope this makes sense. It actually isn't that difficult once you "get" the routine and your plants will love you for the CO2.

 

[Biotoper]

Thanks, guys. Ok, I think I'm getting it.
1) Ignore my KH test kit and the CO2 charts
2) Compare pH in tank to aged bowl of tank water.
3) Aim for a difference of 0.6-1.0. If low, I need more bottles. If high, fewer bottles and remove my carbonate rock.

One remaining question - What should I do about the daily pH swings I'm seeing? Is the 4.5 dKH I was shooting for with baking soda too low given the presence of other anions?

 

[jsoong]

Steve's idea of looking for a pH difference will work ... but you will underestimate the amount of CO2.

Depending on the buffers present, this may or may not be a problem. A weak buffer (like the Ca(OH)2 example) will not dampen the pH change by much so will work. A strong buffer (or a lot of it) might dampen the pH change significantly, so you will have more CO2 than you think. A bit of experimentation therefore will be in order .... I'd aim for the low end of the pH change (at least for start).

Biotoper - regarding #3 - "remove my carbonate rock" - altering HCO3 level is NOT the way to alter CO2 level! Remember you are estimating CO2 levels by an indirect method. You are trying to keep the HCO3 level (& all other buffer levels) the same & estimating CO2 level based on pH change. If you alter the HCO3 level, you will have pH change, but this no loger reflect a change in CO2 level....

In summary, Steve's method of estimating requires:
1. keep everything constant.
2. Inject a certain amount of CO2 to maintain a certain level of CO2 & reach equilibrium <amount CO2 injected = amount of CO2 lost>
3. measure the pH difference. <The exact number vary with buffers present - direct measurement of CO2 or experimentation needed to find the actual number>
4. maintain this pH difference & you should maintain the CO2 level in #2.

Note that if you change anything in the tank parameter - you are back to square 1.

 

[Biotoper]

Yeah, I understand carbonate rock is not a good way to control CO2, but I have some in my tank to keep hardness up (although now who knows what my hardness is) and you said it might start contributing significant amounts of CO2 to the tank as it dissolves.

So I took a bowl of tank water Monday night, and then compared my tank and the bowl last night and again this morning.
Tuesday night: bowl pH 7.9, tank pH 8.3
Wednesday morning: bowl 7.9, tank 7.9
So it looks like my CO2 injection is doing nothing, with my tank at ambient (~3 ppm) in morning and dropping below ambient, presumably to plant uptake, during the day!

Also, I left the pH samples out this morning (both at 7.9), and when I got home tonight, one (I didn't label which) had dropped to 7.7!? Can you explain how a capped vial of water would drop in pH, and why one would and the other wouldn't?

Thanks, Ryan

 

[jsoong]

Yeah, I understand carbonate rock is not a good way to control CO2, but I have some in my tank to keep hardness up (although now who knows what my hardness is) and you said it might start contributing significant amounts of CO2 to the tank as it dissolves.

Just to be picky for a moment - to avoid confusion later - your rock won't contribute CO2 to your tank. Your rock, when dissolved, will increase your KH, which will make you think you have LESS CO2 than actual <if all you are looking at is pH change> & you are tempted to add more CO2 to bring the pH down ......

This is mostly a problem with CO2 cylinder controlled by pH meter. At pH of 7.8, your rock won't dissolve much, but when you add CO2, pH drops & the rock starts to dissolve & raises the pH. Your CO2 controller pumps in more CO2 .... pH drops .... rock dissolves ... pH rises .... more CO2 ..... etc. <http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm - see this for more info>

*** scratching head ****

Now for the fun part ... those numbers you observed are confusing. It seems your pH is dropping all on its own just by exposure to air. It would be interesting to see what the pH will do if you take a bowl of tap water & exposing it to air for a few days.

I am thinking that perhaps the buffer your water Co added is degrading over time ..... Come to think of it, if you add Ca(OH)2 to water:

Ca(OH)2 -> Ca++ + 2OH-
CO2 + H20 -> H+ + HCO3-

H+ + OH- -> H2O
Ca++ + 2HCO3- -> Ca(HCO3)2 .... which precipitates out! <"hard water ring">

In theory, atmospheric CO2 would be enough to precipitate out all the Ca(OH)2, given enough time. Injecting CO2 would just speed up the process..... Now this is just theoretical conjecturing. I wonder if someone with a fish tank using your water has experienced unstable pH ....

It would be interesting to see what the tap water would do on its own (maybe just an airstone to make sure it is equlibrating to air) ... would the pH gets back to 6 (as in the reservour before treatment) on its own?

 

[black udder]

After reading this thread I'm hoping I can hijack some advice.

I'm working on DIY CO2 system (2 2 liters) for my 46g tank.

My problem is that I have about 1-2ppm phosphates in the tank (it's coming from my water supply).

This, of course, is leading to false readings from the KH/PH test. KH was 5 and PH is between 7-7.2 (I think).

Any suggestions for me to try?

Also, Biotoper - what are you using to measure PH? I'm using the vials but I don't know how you're getting such precise readings from the color bars?

Does anybody use the digital PH testers? Are they worth it? I was wondering if it would help since it would give me a more accurate PH count - but the more I read, the more I'm thinking the problem is the inaccurate KH and that's due to the phosphate.

 

[Steve Hampton]

After reading this thread I'm hoping I can hijack some advice.

I'm working on DIY CO2 system (2 2 liters) for my 46g tank.

My problem is that I have about 1-2ppm phosphates in the tank (it's coming from my water supply).

This, of course, is leading to false readings from the KH/PH test. KH was 5 and PH is between 7-7.2 (I think).

Any suggestions for me to try?

First, 1-2 ppm of phosphates will not be the cause of your KH test kit interference. A very large number of aquatic gardeners add phosphates to their tank in to maintain 1-2 ppm, and get accruate KH readings. Your KH interference is coming from another source, either another buffer or more likely another acid present in your tank water. You are getting a rather large amount of KH interference, your KH should be around 1-2 degrees not the 5 your KH kit is showing. I hate to put your through the hoops again, but is the KH in your tank the same as your tap (source) water?

 

[black udder]

Did the tests again:

KH from Tap registers 1 (1 drop turns blue, 1 more drop turns yellow)
GH from Tap registered 7 drops (125.3 ppm)
PH from tank might be 7.3-7.4 (I did the high PH test for giggles and it's an amber)
The normal PH test registers light blue
KH from the tank registers 4 drops or 71.6
This would yield a possible level of CO2 of around 4.8
Phosphates from the tank and the tap were both less than 1ppm

Does this sound more in line?

The tank has gravel, a little substrate, but it's from the LFS, nothing exotic. Lots of plants and the fish. Some ornaments, but I believe they're all plastic.

 

[Biotoper]

To answer your question Black Udder, I'm using the high range pH vial color chart kit (Aquarium Pharmaceuticals master kit to be specific). The color chart is in intervals of 7.4, 7.8, 8.0, 8.2, 8.4, 8.8. My water's mainly varying in that 7.8-8.4 range where it happens to be most precise (smaller intervals). When I report non-chart numbers like 7.7, that's throwing in some guess work (it's slightly yellower than 7.8), but of course color matching is all guess work :| I've read people having huge discussions about how consistant drop sizes are, but I think the big error factor of these kits is just matching colors accurately. I usually try to get about 3 of us to squint at it together and come to a consensus.

Anyway, I'm going to get around to testing a bowl of tap over time to see if the buffer is degrading. I'm also planning to head to one of the Boston Aquarium Society meetings to get some local advice.

To give an update to my tank, who knows if my 4x2L CO2 with Powerhead/Vacuum tube reactor is doing much, but my Val and Watersprite have taken over my tank. I gotta get a pic of my main watersprite - it's floating with just a massive root mass trying to get a foothold into the substrate 18" below.

 

[black udder]

I've had the DIY CO2 in for about a week now and it's seems to be fine. Testing (if I read the colors right) to be about 19ppm CO2.

No reactions from the fish yet (stressing out and the like) plants are going crazy.

I haven't done any aeration either. Just let it bubble away 24/7.