The tank is drilled bottom 2x, left side overflow / right side water inlet. The sump, water flows from top of tower (drilled) 2x - to a drawer which holds a filter pad so you can replace dirty pad,then to 2 sections of media, then returns to tank thru drilled side of sump to an external iwaki pump. There is also 3-4 gals of extra bio balls at bottom of sump.