Strange water test results --> anyone explain? - The Planted Tank Forum
 
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post #1 of 10 (permalink) Old 01-31-2005, 04:43 AM Thread Starter
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Strange water test results --> anyone explain?

In the midst of setting up(slowly) a 20g tank, I tested my tap water with *new* test kits I just bought.
The first time I tested, the readings were as follows:
pH = 7.8
KH = 22
GH = 21

Then I poured water into a glass and let it sit for about 24 hours. The 2nd test results were:
pH = 8.4
KH = 23
GH = 19

This seems rather odd to me. The difference in the KH/GH could maybe be attributed to margin of error, but the pH was DEFINATELY different. I'll test the tap water again without letting it sit to make sure, but it definately went up.
The water in my area is obviously not good for plants, but I was trying to figure out just how bad it was, and whether the pressurized CO2 that I will be adding would be enough to bring the pH down to around 7.0. I would doubt that and will maybe consider just buying the bottled water, which is pH=7 and KH/GH around 0. I may do something like 80% bottled water and 20% tap, but I'm not sure.
My main question is: Does anyone know why the ph might have gone up like this? I guess its just my lack of a strong Chemistry background that I don't quite understand this, but feedback is welcome. Thanks.
-Ryan

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post #2 of 10 (permalink) Old 01-31-2005, 04:49 AM
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The change in pH is very normal. There are dissolved gases (think CO2) that lower the pH. If you let the water sit then those gases escape.
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post #3 of 10 (permalink) Old 01-31-2005, 01:17 PM
 
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I have this exact same problem. My ph is 8.3 or 8.4 day one out of the tap. When I do water changes I have to use distilled water each week. In my 55g, if I change out let's say 15 gallons, I put in 3 distilled jugs. My ph in the tank, with diy c02, gets to about 7.5-7.8. That's the lowest I've gotten. My plants and fish are still thriving though. The only plant I have that isn't is Ludwigia (red).
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post #4 of 10 (permalink) Old 01-31-2005, 03:57 PM
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My KH usually comes out at 21 and pH at 7.6-7.7. It later rises to 8.4. Like said it's normal because CO2 gas is being released, making the pH rise.

80% pure water to 20% tap sounds ok. This will get you a GH and KH around 4-5 and pH of approximately 7.8. You would rather need a target pH of 6.8 for good levels of CO2.
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post #5 of 10 (permalink) Old 01-31-2005, 05:29 PM Thread Starter
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Yeah. I'll either do the 80/20 idea, or just do 100% bottled water, since I have all the pmdd ingredients, which will help buffer the water.

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post #6 of 10 (permalink) Old 01-31-2005, 05:35 PM
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The 100% bottled water is not exactly a good idea. What exactly is in your PMDD, only plant nutrient like NPK, iron, and traces? There probably is no carbonate buffer and not enough calcium and magnesium to give adequate hardness.

I'm not so sure how you concluded your tap water is bad for plants...anyway, mixing your tap with bottled water vs. 100% bottled water will save you time and money.
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post #7 of 10 (permalink) Old 01-31-2005, 05:46 PM Thread Starter
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The pmdd is Epsom salts(MgSO4), Plantex CSM+B, KNO3, K2SO4. Point is, the KH/GH of the water should be raised at least a little to protect against getting pH swings from the pressurized CO2....right?

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post #8 of 10 (permalink) Old 01-31-2005, 05:54 PM
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Quote:
Originally Posted by ringram
The pmdd is Epsom salts(MgSO4), Plantex CSM+B, KNO3, K2SO4. I think that's it. Point is, the KH/GH of the water should be raised at least a little to protect against getting pH swings from the pressurized CO2....right?
Exactly. But there is nothing in your PMDD mix that will raise KH (for buffering the pH) and GH. The MgSO4 will raise GH a very little, and GH should also be made up of calcium, which is not in your mix. Fertilizers are not made to fluctuate the GH/KH.

So you can use 100% pure water and then spend money on commercial addivites like RO right or Seachem Equalibrium. Or go through the extra routine of measuring and adding CaCl, MgSO4, and baking soda to your water.

Or do it the easy and less costly way and mix tap water with the bottled water.

Unless there is some terrible nasties in your tap that we don't know about. (?)
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post #9 of 10 (permalink) Old 01-31-2005, 05:58 PM Thread Starter
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I may just go the route of using 80/20 or 90/10, depending on what the test results are for each of those. I'll try to keep the GH/KH around 3-5, which I hear you don't want to go below.
I also plan on using Eco-Complete, which I hear has a slight buffering capacity, although maybe I don't want to rely on this as the chemicals it may leach into the water will dissipate over time.
Thanks for your help Rolo.

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post #10 of 10 (permalink) Old 01-31-2005, 07:07 PM Thread Starter
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Just for the record. I guess I skimmed over this quote before, but I decided my tap water is bad for plants, based on the tests I did.
pH 7.8+
KH ~ 21-22
GH ~ 19-21
This is too hard and alkaline for most plants. Even with the added CO2 I wouldn't expect it to drop very much since it has a very high buffering capacity. I'm planning on growing plants mostly from S America region, which would require a pH=7 at the *highest* based on everything I've read. Most plants like the pH to be 6.8 - 7.2 from what I've read. Maybe I'm wrong.

I'm not so sure how you concluded your tap water is bad for plants...anyway, mixing your tap with bottled water vs. 100% bottled water will save you time and money.[/QUOTE]

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