PH Problem

Hi,

I just finished my new pond in the spring. EPDM liner, 12 x 10 x 4'6".

I added plants which are doing well and my kids wanted some fish. I want to
put Koi in the pond.

I did some reading and realized that 0.1% salt in the pond was the first
thing to do since my PH was fine.

I added the salt up to 0.8% and noticed that my PH went through the roof. I
have been adjusting the PH by adding Muriatic acid. The PH drops
temporarily and then begins to rise. When the PH reaches 8.2 or higher I
add more of the acid. Every day for the last week I have been doing this.
What in my pond is causing the PH to continue to want to go high? I have no
concrete and only plants with unscented kitty litter as the soil. In all
there is approximately 18 kgs of kitty litter.

Rob