anyway look at the following which i scammed from here http://www.chelonia.org/Articles/waterchemistry.htm
I guess the possiabilities are you have very low co2 levels, the pagoda stone is bumping it up or the sand. how is you plant load? does the ph go up as the KH goes up?
Carbonate Hardness (KH)
This is an area where many people get confused. One of the reasons the term alkaline is avoided a bit and the term Basic is used for the pH scale is because this reading is measuring the alkalinity of the water. It is not the same as alkaline.
The alkalinity is a measurement of the waters buffering ability, or its ability to absorb and neutralise acid. Clearly the more alkalinity or the higher the Carbonate Hardness of the water the less likely you will incur pH swings in the water. It is therefore important to get this figure reasonably high to stabilise the water.
Exactly how high you want your dKH (degrees of KH) will depend on what pH you choose to use. To people keeping fish from the African Lakes this is the life and death of their fish. Those fish live in very stable high pH conditions, they do not like change and the conditions can be difficult to imitate. Fortunately turtles are not quite so sensitive. However, I do think that basic fish keeping can teach the aquatic turtle keeper a thing or two about water.
Ways to increase kH
Adding sodium bicarbonate (baking soda). One teaspoon of baking soda added to 50 liters of water can raise the kH of the water by approx 4 deg dH without a major affect on pH.
Adding an air stone to increase surface turbulence driving off carbon dioxide (CO2)
Adding commercially available products to increase buffering capacity
Ways to lower kH
Injecting carbon dioxide (CO2)
Use reverse osmosis (RO) water. You can mix tap water with reverse osmosis water to achieve the desired kH.
Adding commercially available products to decrease the buffering capacity.