I remember there was a couple people who made their own carbon filters, using a refillable cartridge t'd off their return using a Y and Jaco fittings. I had a bad experience using filter bags filled with carbon( I had one get caught in a pump and shredded it, sending tiny pieces of carbon all over my tank) Plus I think it may be more efficient than just a bag sitting in my sump. If you have built one, do you have pics, and/or a list of parts and suggestions to make it work the best? I think it's an ingenious idea and I'm a neat freak so I think it's something I want to try.