Ok I found the proper link. Quite confusing layout.
I think the design is a mistake. Plant filters are
not the way to go IMO. I think the bottom drains to one side is a bad choice as well. I assume you will have a sloping bottom with the drains in the deepest part. It doesn't work that way I'm afraid and you will end up with a slope covered in shite that can't/doesn't get to the drains. I'm sure you have put alot of thought into this but I'm afraid the ideas you have had are all a bit far down the learning curve. Sorry, I know you want to do it right but this isn't the way
B.Scott