water data from purification plant

and?

pH does look high, but I'd test an aged sample of your tap water before I got all worked up. A lot of utilities drive up pH to keep the water from corroding pipes.

If you have low KH and high pH, the utility probably treats the water using the lime-soda method. A good descripton of that is found at the Skeptical Aquarist (http://www.skepticalaquarist.com/docs/filtration/softening.shtml). Look for the section on Soft water with high pH .

Checking for pH after aging the water (sitting out overnight in a shallow bowl or aerating the water vigorously for 30-60 minutes) will give you an idea if your water pH will actually stay that high.

HTH,
Jim

It's actually kinda surprising that there's no hardness reported. Mine reports both hardness and alkalinity, both total and carbonate.

Almost forgot, definitely do an aged water test. This will tell you the pH you'll see in your tank.

If I'm interpreting the numbers correctly (jump in here, happychem!), it looks like you don't have a lot of buffering. I bet you a nickel that if you age a sample of water you'll see a goodly drop in pH.

Along those same lines, I'd be concerned about the water's ability to hold pH in the presence of acids and bases... this might be case for some mild boosting of KH to provide a bit more buffering.

HTH,
Jim

Yup, definitely looks like parameters from lime softened water.

The process adds extra CO3--:

Ca++ + CO3-- = CaCO3

With the increase in CO3--, CaCO3 equilibrium moves right, precipitating CaCO3.

A strong base (Ca(OH)2) is added to prevent an equilibrium shift in:
(simplified)
CO2 + H2O = 2H+ + CO3--

By removing H+. Likely the base is added in excess, so not only does this equilibrium not move left, it probably moves right, further contributing to step 1.

Now you've got water with a high pH and little CO3-- (or HCO3-, for that matter).

Age your water, all these changes monkey around with CO2 equilibriums, it's likely that you'll observe a lower pH after the water re-equilibrates with CO2.

In any case, you're still going to have water with very little buffer. That's okay, it just means that you have to be attentive and diligent in your water changes.