I use Fluval peat fiber in my canister filters, placing it first in the
appropriate size media bags. I tried regular peat moss once, but it is
rather fine and compacted too much in my filter after being wet a while,
not to mention loosing a lot of it into my tanks when I started the
filters. (the peat moss works pretty good as a substrate conditioner,
however). Fluval also has a peat granule which is supposedly very
concentrated...but so is the price! 70 grams of peat fiber cost about
$4.50 mail order, or, (at least in my area) about $7.00 retail. I just
buy a bunch of it when I order. To me, $4.50 seems inexpensive compared
to the carbon I was using previously! :-)
Jim