Something doesn't sound right. At a pH of 10, hardness solubility should not be 100 ppm as CaCO3; certainly not as ion. Alkalinity could be this low, but would likely be higher unless you are pulling from a lime deposit sand. Hardness at twice the alkalinity indicates calcium chloride or sulfate, or magnesium salts of the same. High levels of non-carbonate hardness would typically be the case at pH ranges nearing neutral, not above 9.0.
The pH at this level is detrimental to fish growth and health. Natural waters do not stay at this level unless they receive additions of lime, caustic soda, or soda ash. Lime and soda ash are naturally found soil deposits in some areas. I have not heard of that being the case around your area of Texas. The only area in Texas I am familiar with that has wel water that high in is water is near College Station, where well water runs 9.5 pH, but low hardness from the natural soda deposits. This water is extremely soft (low in hardness)from the high pH and high sodium content (solubility of hardness salts at 10 pH and ambient conditions is less than 60 ppm due to retrograde solubility).
I would check the pH out of the well. If it is this high, you will need to add sulfur salts such as ferric sulfate to lower pH with out lowering alkalinity. Once the pH is lowered, CO2 solubility will increase, and alkalinity and hardness levels will stabilize.
