so where do you get these? where a box is over a opening of the tank and the pump pumps water into it and it goes down back into the tank.
http://www.aquarticles.com/articles/management/Barrett_DIY_Overtank_Tricklefilter.html
http://www.aquarticles.com/articles/management/Barrett_DIY_Overtank_Tricklefilter.html