my lfs showed me that I should once a month put about 1 1/2 cup of carbon in a bag and throw it in my sump. 225 gallon tank.That the way they mantained it before I took it over. There is no pressure from the water, so it does the slow thing. That is what I have been doing for a long time now, I just read that after awhile,the carbon will release all of the impurities back into the tank. Now, I am a little confused. I know that if I put the carbon in a high flow area that I should only have it for a few days of so, then take out. What is the deal? I'm still learning. 