add 1/2 a teaspoon and test, if its not enough add a bit more a test again, if its still real high on the first test after adding it you can add more than just a little, another 1/2 tablespoon should be alright, repeat till you hit the 6.0-7.0 range, preferbly in the middle. actually wait, you said 8.0, so i would add a full one right away the first time and then test and then start adding little bits and re-test if its not quite there, a teaspoon should drop it about 1 full level be about per gallon of water.(so a gallon of about ph 8.0 water should drop to around 7.0) then just flush the soil with 2-3 times as much of properly ph'd water as you have soil in the pot.
