# How can water with a very low hardness have substantial buffer capacity?

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)

• Is your tank exposed to the atmosphere? Carbon dioxide can dissolve in water to form a fairly acidic solution. Jun 14, 2014 at 22:40
• 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.
– LDC3
Jun 17, 2014 at 0:52

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.
• @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. Jun 16, 2014 at 5:18