using acid to lower GH, KH, pH? - The Planted Tank Forum
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post #1 of 19 (permalink) Old 03-05-2008, 01:26 AM Thread Starter
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using acid to lower GH, KH, pH?

I am in a high school ap chemistry class learning about acid-base reactions. I am wondering whether or not I can use H3PO4 (phosphuric acid) to lower such parameters as kh, gh, pH or even TDS because when H3PO4 disassociates into H+ and PO4-; Ca+ and Mg- should react with the aqueous phosphate ion to produce insoluble Ca3(PO4)2 or insoluable Mg3(PO4)2. the free H+ ions would lower the pH and any undesolved phosphate ions could be considered fertilizer right? Show me my faults because I think that I probably didn't think this through very well.
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post #2 of 19 (permalink) Old 03-05-2008, 01:44 AM
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Phosphate (PO4-) is an important ingredient of genetic compounds, but there's no guarantee that it will be the lone end result of adding H3PO4 to your tank. And even if that was all that is produced, it can (will) react with metal oxides and form ions that are insoluble and can not be used by your plants. And creating high phosphate levels is pretty much asking for an algae bloom.
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post #3 of 19 (permalink) Old 03-05-2008, 03:03 AM
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And creating high phosphate levels is pretty much asking for an algae bloom.
This has been proven to be untrue, but I argee this may cause unwanted effects. Why do you need to lower your pH, and how much do you want to lower it?
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post #4 of 19 (permalink) Old 03-05-2008, 03:19 AM
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Obviously, he could simply use r/o water to soften, I think he's asking as more a chemistry quiz. Please quote your source on PO4 levels not encouraging algae growth. That counters not only everything I've ever read, but all of my personal experience and working at tropical fish stores. To grow strongly, film algae needs phosphate levels over .03 mg/liter. I've tested hundreds of people's tank water with algae problems, and very very rarely have levels been below this. Cause may not equal effect, but there's too much of a correlation for this to be coincidence.
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post #5 of 19 (permalink) Old 03-05-2008, 03:21 AM
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many people here keep phosphates at like 3 ppm...pm plantbrain if you want a good explanation...
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post #6 of 19 (permalink) Old 03-05-2008, 01:42 PM
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Originally Posted by snakeskin View Post
I am in a high school ap chemistry class learning about acid-base reactions. I am wondering whether or not I can use H3PO4 (phosphuric acid) to lower such parameters as kh, gh, pH or even TDS because when H3PO4 disassociates into H+ and PO4-; Ca+ and Mg- should react with the aqueous phosphate ion to produce insoluble Ca3(PO4)2 or insoluable Mg3(PO4)2. the free H+ ions would lower the pH and any undesolved phosphate ions could be considered fertilizer right? Show me my faults because I think that I probably didn't think this through very well.
Phosphoric acid (H3PO4) will lower pH. You might also precipitate out some of the calcium and magnesium phosphate salts. However, the reaction is not complete (an equilibrium), so some phosphate will remain in the water column.

The problem is twofold: scale (1ppm PO4 versus 100ppm CaCO3 equivalent GH) and that the whole system reacts: So lowering the pH (by adding phosphoric acid) also increases the solubility of the phosphate (and any carbonate) salts - more phosphoric acid actually would result in less precipitate and a higher TDS and GH. Also, the kH is a measure of buffering ability - it would be lower, but not in a plant-meaningful way since the (dihydrogen)phosphate is taking the place of the bicarbonate.

If you want to model it, download the program "PhreeQC" (I think it is still around) and put in your parameters.

As far as phosphate and algae, I just brought my tank up to 2ppm phosphate since I was getting a lot of green spot algae on the glass - the GSA will probably not come back (my past experience).

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2g Mac-quarium. Clown gravel, fluorescent plastic plants, and 2 guppies.
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post #7 of 19 (permalink) Old 03-05-2008, 11:35 PM Thread Starter
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wow I liked that last one! Thanks KevinC
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post #8 of 19 (permalink) Old 03-05-2008, 11:38 PM Thread Starter
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This has been proven to be untrue, but I argee this may cause unwanted effects. Why do you need to lower your pH, and how much do you want to lower it?
I have a pH of 8 in an "Amazon Tank" and want it to provide the water that is appropriate for the fish. I only want neutral water.
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post #9 of 19 (permalink) Old 03-05-2008, 11:39 PM Thread Starter
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I have seen some powders that claim to neutralize the water. I know I should not try and screw around with the chemistry but do any of you think this is a good idea?
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post #10 of 19 (permalink) Old 03-05-2008, 11:45 PM
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I have a pH of 8 in an "Amazon Tank" and want it to provide the water that is appropriate for the fish. I only want neutral water.
Use RO/DI water to dilute your tap water. Discus also need a low gH and kH.

What "powders" are you referring to?





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post #11 of 19 (permalink) Old 03-09-2008, 06:27 PM
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I second that motion. Use RO deionized water to lower general and carbonate hardness. If you really want to maintain a specific ph then set up an automated co2 injection system.


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post #12 of 19 (permalink) Old 03-10-2008, 09:12 PM
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Use RO/DI water to dilute your tap water. Discus also need a low gH and kH.

What "powders" are you referring to?
Discus do not need low GH/KH I have a freind who breeds them in tap 7.6 ph and GH at about 8...Fish need stablility more than a certain paramater being exatcly what it is "supposed" to be.
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post #13 of 19 (permalink) Old 03-27-2008, 08:44 PM Thread Starter
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I have heard that discus in lower pH environment do better because their slime coating is produced easier in that environment. I have also heard that pH and GH do not affect discus as long as they are not extreme because discus have been breed for many years in domestic conditions.
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post #14 of 19 (permalink) Old 03-27-2008, 09:19 PM
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It all depends on the source of the fish in question. Wild-caught discus will indeed need a lower pH, gH and kH.

The point I was making is that if you're attempting to re-create the Amazonian conditions where wild discus are found you also need to lower the kH and gH, not just lower the pH.





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post #15 of 19 (permalink) Old 03-28-2008, 04:09 AM Thread Starter
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It all depends on the source of the fish in question. Wild-caught discus will indeed need a lower pH, gH and kH.

The point I was making is that if you're attempting to re-create the Amazonian conditions where wild discus are found you also need to lower the kH and gH, not just lower the pH.
My fish is not wild caught, but the rummynose tetras that live with him are and the rest of the fish in my aquarium are generally considered "amazonian" with the exception of my flagfish. The rummynose tetras have a vibrant red hue to their noses and are acclimated to my water conditions but the point of the question really was just to see if neutralizing powders were worth adding in order to create a more idealic water chemistry.
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