Added a three stag filter system...water comes into the can on right thru bottom (it is filled with lava rock) exists at the top...can 2 is 2/3 full of "bio-balls, 1.25 inches"....found 2000 for $100 on ebay....the last can is a settling can and then it is pumped up into the fish tank. The can on the stool is water for topping off the system after all traces of chlorine have burned off.