i'm a new bee can u tell me what is HOBs. this forum rocks! another question for my fluval 305 canister filtration system i'm having trouble finding out what type of media to get to put in my trays, rite now i hav carbon, the ceremic rings, & the foam sponges there is 1 basket left, what do i put in it. i have a 55 gal. tank with 5 very aggressive cichlids 2 which are @ least 4-5".