Filter Design

This is probably a better question for alt.fishtanks, but I figure I know
you people better, and I live it here...

I am thinking of experimenting with a DIY filter for my 55gallon tank. I
figure working on small designs may yeild some fun project for the pond.
Anyway, does anyone care to discuss some designs?

My tank sits on a table of sorts and is 3 feet or so off the ground. There
is a staircase behind it so building a filter above it, and letting it
gravity feed back into the tank could be problematic. Letting the tank
gravity feed down to the filter and then pump back up to the tank would be
problematic (I think?) because then the tank can conceiveably drain.

So where do I go from here?

BV.
www.iheartmypond.com