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Discussion Starter · #1 ·
What effect, if any, does calcium hydroxide have on the CO2 levels in an aquarium?

The City of Dayton "softens" water with 'lime' and I assume it is slaked lime. I'm not sure of the science behind softening water this way but the total alkalinity is reduced from 270ppm to 72ppm, Ca is reduced from 227ppm to 70 ppm and Mg is reduced from 43ppm to 17ppm. The pH of the water is increased from 7.36 to 8.49 during this process and 25ppm of CO2 is stripped from the water. 6 ppm of Phosphate alkalinity are also added to the distribution system.

Since stopping the use of RO water this summer, I have had a constant battle with BBA on my substrate (Eco Complete) and filter intakes. Healthy plants remain unaffected, though most new plants I add to the tank develop BBA until new growth is formed. Even though I have good nutrients and seemingly high CO2 levels, I cannot help but think the "softening" of the water somehow makes the KH/pH relationship invalid.

 

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This is an educated guess. CaOH give a net charge of 1 postive ion; Ca=2+ and OH=1-. Therefore, I believe that because you have and excess of postive ions in your water, it will combine with the carbonic acid in your aquarium produced by your CO2 system. The effect will be a gradual increase in your pH and kH, and it will lessen the amount of carbon in your tank. I would try increasing your CO2 output.
 

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Discussion Starter · #4 ·
Thanks for the response Donald. I found an explanation on the use of slaked lime in water softening here: http://www.freedrinkingwater.com/water-education2/410-lime-soda-water.htm

This is the type of site I was looking for since I don't know enough about chemistry to read most of the other sites ;) The first sentance of of the fifth paragraph seems to spell it out for me. Since slaked lime removes calcium bicarbonate, I would assume it removes other forms of bicarbonates also.

If the site is correct, it would explain the problems I have been having with seemingly high CO2 levels (190-200+ppm) with no apparant fish stress. It would also seem to point out the reasoning as to why the BBA started after I quit using reconstituted RO/DI water.

I do believe it is the bicarbonate portion of alkalinity that the KH/pH relationship relies on. Someone please correct me if I am wrong on this aspect of the KH/pH relationship
 

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Not a chemist, but I took enough college courses in it to be dangerous. Adding Ca(OH)2 to the water will do many things to the water to make it less desirable for an aquarium.

First of all donaldmboyer's chemistry reasoning above is incorrect (no offense). There simply cannot be an imbalance in + and - charges in ionic species. Calcium hydroxide exists as Ca(OH)2 instead of CaOH for that reason.

Calcium hydroxide is a VERY potent base (opposite of acid). Think of something on the order of Drano, which is NaOH. This, by itself, will neutralize any acid that was hanging around and raise your pH significantly. If they've added enough Ca(OH)2 to get the pH above 8 the carbonate buffer system that the CO2 chart relies on is completely overwhelmed. When you then add a relatively weak acid (CO2), the pH will drop more quickly than the chart says it should (at first). Once you've added enough to get closer to 6.3 the carbonate buffer system becomes more 'active' and pH changes will be less dramatic as you add even more CO2. There is enough 'magic' chemistry taking place in your water to render the chart (and any conclusions about how much CO2 you have) completely useless.

The more concerning thing to me is the final process described in your referenced source. The final step of adding soda ash (Na2CO3) causes Ca++ and Mg++ to precipitate out of solution. The net result is a LARGE addition of Na+ ions (maybe 200 - 400 ppm), which plants DO NOT LIKE. This might set them back enough to let the BBA get an upper hand. This is the same reason you can't simply get 'soft' aquarium water from regular home water softeners. They use NaCl (table salt, rock salt, etc.) to swap Na for the Ca and Mg in the water.

Be careful when thinking about what the city is adding. Their goal is to reduce Ca and Mg. This affects GH. All the trick chemistry might reduce levels of KH too (buffering capacity), but this is not their intent. Nobody but us complains about low KH. BTW, your plants don't need carbonate. A few species are pretty good at slowly getting carbon from carbonate when they need to, but it is MUCH easier (and faster) to just pluck off the carbon from all the CO2 we're injecting. We only add carbonate to create a buffer to the acid we intend to add. If your carbonate level (KH) is low, you can still add some baking soda (NaHCO3) (too bad the Na comes along, but ppm levels aren't all that high).

Honestly, I'd go by whatever bubble count you think is appropriate for your size tank. I have a 46 gal with a KH of 4.5 and I use about 140 bpm from a Milwaukee regulator to get about 30 ppm CO2 (I think). The phosphate buffer that Dayton gives you for 'free' (they add this to slow down corrosion in their expensive pipes) will play into this as well (and not in a good way). I'm a little shaky on all the details of the chemistry here, but I know enough to know that you have 'problem water'.

Find out what the Na level in the water is. I bet that it's pretty high.
 

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Discussion Starter · #6 · (Edited)
I got a Water Quality Roport back in September after hounding the City for it for nearly 9 months. I listed a few things in spreadsheet form so I could upload it if need be. Not sure how pretty the spreadsheet is but here it is... I found a few errors while double cheking the report so I will correct them and repost...
 

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Discussion Starter · #7 ·
Here is the corrected version of the spreadsheet. I'm not sure if Dayton uses the Soda Ash part of the softening and all I really know is that they use "lime" I don't have any more specifics than that. They do add enough Calcium Hydroxide to get the pH to 8.59 as you can see in the chart.
 

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Huh. Guess the city doesn't exchange with soda ash. My town says their Na levels are 15 ppm for comparison. Your level isn't all that high.

I'm curious what your tap water pH is after you let it degass for a day or two. You probably posted this somewhere but I can't find it. If your pH is still quite high (for your KH) you could correct it with hydrochloric acid.

Ca(OH)2 + 2HCl --> CaCl2 + 2H2O

This would easily lower the pH to where it's 'supposed to be'. The only byproduct would be CaCl2 which isn't such a bad thing. The way I'm looking at this, the excess OH- ions that are causing your pH to be high would be eliminated and you would be back to where the CO2 chart would be valid again.

Come to think of it, maybe I should add some Ca(OH)2 to get my pH up to where it is supposed to be since I have about 0.3 pH units of acidity that I can't acount for (tannic acids, organic acids, etc). Then again, maybe that's a bad idea.

Ever think of going back to the RO water? If you figure out how to get the BBA to go away, ship some of your water to me. I still seem to have loads of it despite trying every trick I can think of.

Like I said before, I know just enough to be dangerous......... Doesn't quite seem right to dump hydrochloric acid into a fish tank.........
 

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Discussion Starter · #9 ·
I tested everything yesterday and posted in another thread. Here is the info cut and pasted from this thread:
http://www.aquaticplantcentral.com/forumapc/showthread.php?t=10863&page=3

Tap
pH - 8.51
KH - 5.59 dKH
CO2 - 1 ppm
TDS - 218 ppm

Tap aerated/rested
pH - 7.45
KH - 5.59 dKH
CO2 - 4 ppm
TDS - 218 ppm

This is what seemed strange to me. I believe most Tap water pH results rise when the sample is aerated and the CO2 is driven off. Mine appears to be just the opposite, though the CO2 of 4 ppm is about right. The tank water seems to follow along with the same 4ppm of CO2 after aeration so I assume there are no acids in my tank playing with the chart.

Tank
pH - 6.15
KH - 6.08
CO2 - 126 ppm
TDS - 212 ppm

Tank aerated/rested
pH - 7.67
KH - 6.08
CO2 - 4 ppm
TDS - 212 ppm

Being the anal type that I am, I also tested my RO/DI water...

RO/DI
pH - 6.35

RO/DI aerated
pH - 6.26

KH is assumed to be zero on the RO water and the TDS is 1 ppm.

I initially used RO water since we have a whole house water softener. I switched it over to KCL after about 3 months of living in the house, then realized that the kitchen faucet (cold side) is not hooked up to the softener. It was about this time I went back to TAP water. I figured the extra K wouldn't hurt ;)

I actually went back to the RO water this Sunday but I'm slowly making the change. I did a 25% wc with RO buffered to 3 dKH on Sunday and another 25% straight RO on Tuesday to try and reduce the GH and KH a bit. KH had risen to 9 dKH since I have been adding 3ppm of Sodium Bicarb with each water change to try and add some bicarbonates back to my water. Pearling has increased since the addition of the bicarbonates. Tuesday's wc only included KNO3 and KH2PO4. Traces have been dosed normally.

Your statement about getting the pH below ~ 6.3 is correct. Quite a few of us around this area have quite high CO2 readings. In order to drop my CO2 much past where it is now, I prety much have to have a stream of CO2 going into the tank. No way to even begin counting the bubbles, just a stream of CO2 to get the pH to drop. Seems very wasteful even though CO2 is cheap.
 

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Threads like these make me insecure about all the things I've ignored & taken for granted before. It really seems like there should be a straightforward recipe for success. Then again, if you didn't need to tinker with things what fun would it be? I don't think too many people in this hobby are content to just sit and watch fish swim around the pretty plants.

I have a friend who is a Chemistry PhD @ Cornell. Maybe I'll ask him about all this CO2/pH/KH stuff.
 

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Discussion Starter · #11 ·
guaiac_boy said:
Threads like these make me insecure about all the things I've ignored & taken for granted before. It really seems like there should be a straightforward recipe for success. Then again, if you didn't need to tinker with things what fun would it be? I don't think too many people in this hobby are content to just sit and watch fish swim around the pretty plants.
I know what you mean. Before moving here I had well water that was very stable. Now that I am on City Water, it is hard to say what they do to the water :) Tinkering is part of the fun once you figure out what is wrong but it can be aggravating until you solve the problem.

guaiac_boy said:
I have a friend who is a Chemistry PhD @ Cornell. Maybe I'll ask him about all this CO2/pH/KH stuff.
I would appreciate that :D This has been driving me crazy for a while now and I would love to get to the bottom of it. I thank you and the rest of SWOAPE will probably thank you too if we get to the bottom of our CO2 issues.

And thanks for the help so far, it is greatly appreciated!
 

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They are doing that partial lime softening, when I was there and you mentioneed the KH/GH, I was suspcious.

They dump the lime in, raise the pH to 10.1, that preciptates out the CaCO3, lowers the KH and GH.

But makes the shower soap bubbles extra bubbly:)

KH/pH=> KH can and is influenced by hydroxides, that is why you have such squirrley CO2 readings.

Hope that helps,

Tom Barr
 
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