robv60,

I use oyster shell all the time. I put a tray of it in my filter and
let the water fall throught it as part of the filtering process. If
you can't do this with the filter you have, I have in the past made a
bag from nylon netting you can get at the fabric department of your
local Wal-Mart, or whereever, and place it where the water will run
over/through it. The bag does not have to be large, but the small it
is the more often you should check it.

Tom L.L.