Peat in an aquarium to lower pH?

Hi, I have a 55 gallon with 4 discus in it, and I have pretty hard, alkaline water. I don't like using chemicals to lower pH or anything, but I know lots of people use peat in the filter to bring down the pH, and I thought I should try it. I just don't know what kind or how much to use or anything. I'm afraid I'll buy something toxic on accident. Could anyone suggest what kind of peat I need to buy, and how much of it to put in the filter?