Question
We put a 3500 gallon pond in last year and watched the alkalinity and hardness drop as the year progressed.  I just took my first sample of the year and found the pH @ 7.5, but the GH is 40 and the KH is 40.  Our city water is relatively soft as well so water changes don't help alot.

How can best increase and maintain alkalinty/hardness to is improve the buffering capacity??

Answer
Simple - add baking soda.  Baking soda or sodium bicarbonate is a good buffer but won't raise the pH too much since your pH is already 7.5.  I have soft well water.  I add baking soda to my 1800 gallon pond each week.  How much?  I don't measure (I'm bad).  I probably put in 1/4 a cup, not much.  I should add more.  To get the right dose, I suggest mixing a cup with a bucket of water and spreading that around.  Wait a day.  Measure the pH, alkalinity, and hardness to see how they've changed.  It should not effect the hardness but may raise the pH a tad.  Good bacteria and other natural processes use up the alkalinity so as the pond develops with time, the alkalinity goes down. I remember reading some article that said to "feed" the good bacteria with baking soda.  If you also want to raise the hardness, then there are other chemicals you could add but it's best to not alter the pond water too much.  For example, if each week, you have to add exactly a cup of this and that, and then it rains heavily, or you go on vacation, what happens to that exact composition?  It's not going to be stable.  I have soft well water, and my pond animals are doing as well as anyone else's.  Good luck!

Robyn
fishpondinfo.com