Improving my "slippy filter"

[warren]

HI every body,I have been using what I just learned is basically a slippy filter for about 24 years. Last year I decided to improve on it. I ent from 1 large frame box with a pond liner and lots of lavailable rock. I changed to to 2 restaurant food tubs chose because temp range is-40 degrees to 140 degrees so they can handle all years temp. My questions are is there a formul for determining proper gph per cubic foot of media? I am still using lava rock and would like to create a flow through media that would maximize efficiency of media. I s thete a way to create the circular flow recomended in a rectangular box. I did not know any of these things when i bult original. And have to much investment to start over. I though if it ain't broke don't fix it. But since I am rebuiding waterfall let' make it more efficient. I will provide additional flow including small bog to provide total water flow for pond volume. I have three pumps
available. I still need to figure out total volume of pond since building larger shelf and volume lost to lily and lotus pots.I also wonder if adding venturi valves to one or both boxes would help necessary bacteria. Thank you for any advice available

 

[USMC Ponder]

I used this here to build mine

http://www.skippysstuff.com/biofiltr.htm