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&lt; 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&lt;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)
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.