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My freshwater fish tank has a pH of 5.8 and a KH value in excess of 40 or 720+ppm while still maintaining a water hardness of 20 ppm or GH< 1 now I am a math/phsyics student but I have taken both first year chem class at university so I expect to follow most basic ideas pertaining to chemistry but this doesn't make a lot of sense. Most water with a high alkalinity also has a basic pH around 8-8.5 normally I'd say that a high KH value is simply saying I have an acidic solution that is very resistant to a pH change i.e. a large buffering capacity but my water is very soft to the point of almost not having anything in it ( GH<1) or less than 20ppm of minerals how can water be so acidic have such a large buffering capacity and have nothing in it?

(the reason why I am asking here is all the aquarium forums i have tried can't seem to figure out what's going on in my tank)

EDIT:

This answer poseing an intresting possibility my tank has Turtles in who obviously are advanced enough to excreet urea instead of amonium i Titrated out a reaction and Got KH value of 28 and GH value of 4.5 i also added several air stones ( my buddie has a bio chem degree and figured it could of been too much $CO_{2}$ we managed to get the Ph up to ~6.4 and drop the KH value ot be more closely in line with accpetable parameters ( a value of 8 still oddly high compared to the GH value of 4.5)

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    $\begingroup$ Is your tank exposed to the atmosphere? Carbon dioxide can dissolve in water to form a fairly acidic solution. $\endgroup$ – Dissenter Jun 14 '14 at 22:40
  • $\begingroup$ It will take time for the bacteria in the aquarium to adjust to the new pH level. When it does, you will probably see the KH value decrease. $\endgroup$ – LDC3 Jun 17 '14 at 0:52
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Distilled water may have pH 5-6 because it dissolves significant amount of carbon dioxide. Carbon dioxide in water is in equilibrium $$\ce{CO2 + H2O <=> H2CO3 <=> H+ + HCO3-}.$$ Since amount of dissolved carbon dioxide is much larger than formed acid, the solution acts as buffer. Tap/sweet water usually contains fair amount of K, Na, Ca, and Mg cations, that traps significant amount of carbon dioxide forming $\ce{HCO3- /CO3^{2-}}$ buffer, that is alkaline.

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  • $\begingroup$ Are you saying that these cations are attracted to the hydrogen carbonate ion and the carbonate ion? $\endgroup$ – Dissenter Jun 15 '14 at 18:01
  • $\begingroup$ Said cations are dissolved from minerals by exchange with protons produced by mentioned equilibrium. Only limited amount of carbon dioxide can be dissolve in water and be in equilibrium with specified amount hydrocarbonate, and maximum amount of hydrocarbonate in a solution with notable amount of metal cations is determined by amount of cations that does not have other (non-(hydro)-carbonate) anion partners. $\endgroup$ – permeakra Jun 15 '14 at 19:43
  • $\begingroup$ Can you give an example of this proton exchange? $\endgroup$ – Dissenter Jun 15 '14 at 20:00
  • $\begingroup$ @Dissenter for examle, anorthite on prolonged contact with atmosphere slowly gives up its calcium , producing kaolinite $CaAl_2Si_2O_8 + 2 H_3O^+ = Ca^{2+} + Al_2Si_2O_5(OH)_4 +H_2O$. The process occurs at very slow rate, but fast enough on geological scale, and requires only rainwater to occur. Other volcanic aluminosilicates behaves similarly. $\endgroup$ – permeakra Jun 16 '14 at 5:18
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In an aquarium, there could be several non-mineral buffers. The most likely one is a phosphate buffer. There would also be carboxylic acid (amino acids) and probably urea, as well as carbon dioxide.

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