Pepetj's got it right. You can use peat to drop your pH, but you need to be really careful, because it also removes the water's pH
buffer, meaning you can end up with a rapid pH change (which is much worse for your fish than a slightly high pH).
My suggestion for using peat is not to add it to the filters in any way, but to pre-treat your water with the peat. Get a big (new) plastic garbage can and put a pillowcase filled with the organic peat in there. Run an airstone under the pillowcase so it percolates through the peat, and leave for a few days. Then you can pre-mix the amount of regular water to treated water to get the precise pH and hardness that you want.
Yes, this is labor intensive, but it's also much better for your fish in the long run.
Now, I've got a pH of 8.2+ in my tap water, and I have some tetras and glassfish, both of which prefer soft, acidic water, and they're doing wonderfully. They're bright, active, and always hungry. Primarily, the only thing that a high pH will impede in fish like that is breeding.