[Q] Water Chemistry

 I've been lurking about for a few weeks now so I guess its time for an   
introduction: My name is Larry and I'm from Carver Ma. Carver is down   
near Cape Cod.
  I have set up a small pond in my living room, I used a kidney shape   
plastic tub and made a 2 x 4 frame to hold it. This has been going for 6   
years and the goldfish have done well.
 I would like to get some live plants going so I started looking into the   
water chemistry as a starting off point. ( I know that lighting will be a   
major problem but that I'll save for a later posting ).
 My tap water,( it comes from a shallow well ), tested out to be at a PH   
level 6.0 and a general hardness of between 10 and 20 ppm. This was a   
little surprising I would have expected well water to be very hard not   
soft. ( I was so suprised I did the test two times to be sure.) The water   
in my pond tested out to be PH 7.2 and the general hardness was off the   
scale!? There was not enough nitrites to register on the scale so they   
are not a big problem. My guess is some of the decorations added to the   
pond are not a inert as they were first thought to be. I've been   
pondering ( no pun intended ) the situation and I think it may be   
possible to use these agents to my advantage. If I can figure out how   
much water to change on a regular basis I might be able to stablize my PH   
and general hardness at some point. My first question is ( finally I get   
to the point ! ) does anybody have any good formula's for figuring out   
 how much water at hardness x to add to water of hardness y to get water   
at hardness z ect.. also for PH and the Like.
 I'm not sure if it is possible to do it this way so I'm open to all