Are you sure about this leaching business? I mean, whenever I cleaned a solution discolored by contaminants in the lab, we never had a leaching problem, and I used the least amount of carbon possible...it loaded up quickly, but did not release anything bad back into solution, even after multiple passes.
Activated charcoal (carbon, as we call it in aquarium-speak) is remarkably effective at removing contaminants. How fast it loads up and loses its effectiveness depends on how much you use and how much stuff needs to be removed from the aquarium.
That said, I rarely use carbon...I just use the Magnum Micron pleated filter cartridges along with the floss pre-filter in both my Magnum 350's. If you need the carbon, toss in one of those media bags filled with the stuff. There's room outside of the filter cartridge to do this. I've used crushed shells, carbon and so forth in those handy fine-mesh bags.
v/r, N-A
