Yeah, I made a biofilter for my system, kinda like yours. With an IBC tank. I made mine for less than $10 US. The only bad thing about it is the cleaning. So I took it off the system.

Trying to figure out how to create one with a 55 gallon barrel that cleans itself, using a backflow. Here is a link that shows ya some pics on how I made mine. If you can see them.

http://www.facebook.com/home.php?#!/alb ... 1691732449

I replaced the rock with plastic dish scrubbies from the dollar store. It was getting time consuming taking it apart every week and spraying them down. It was easily assembled though.

I am trying to make one like this with a 55 gal barrel.
http://users.vcnet.com/rrenshaw/BubbleBead.html

I dont know why it is hour glass shaped, I do not think it has to be.