I'm not sure why some of my fish are dying. I have a 2 year old tank of Brichardi and some other assorted africans. It seems that all of a sudden at least 4 or 5 of the mid size brichadis have died. These fish were at least one year old. They were offspring of my 2 original Brichardi. The only thing I changed was the sponge in my AC500 filter. I also have a Whisper3 filter and a power head for filtration. All parametes are fine. Ammonia and Nitrites are zero and nitrates look to be between 20-40. Just wondering if anyone has any idea what could have caused it. The tank is a 55gal. Everything looks normal, could it have been the sponge. I rinsed it before putting it in the tank.