Yeah, gotta agree with
jjohnwm
there. Bad bad move placing a sponge in a vertical slot. If it restricts the flow now when the sponge is at it's cleanest, then how do you thing things are going to go as it slowly clogs up!
If you go back to my crappy looking super efficient set up in post #11 I'll explain why I made the decision to put a bucket in that first partition because it addresses the problem you are facing now. I came to that decision by a lot of trial and error, it wasn't always like that.
Originally I had a heap of vertical sponges in that first section, of varying grades, all lined up against the partition glass. But I wasn't maximising the whole area of each square piece of sponge because the water only flowed through the bottom inch or so, thanks to the glass partition! This inch or so of sponge would clog quickly, and the water level in my first section would rise. So I very regularly had to clean them and that leads me to the most annoying part. As I disturbed the sponges and gently lifted them out most of the gunk collected on them would simply fall off and sink to the bottom of my first compartment, which then required syphoning out. Total pita.
I quickly worked out that using the full surface area of the sponge is best for optimum efficiency. How could I do this? My next plan was to place them horizontally. On top of a makeshift plastic platform made of a bit of thin pvc tubing and egg crate. This would keep that bottom gap unobstructed between first and second section. Bingo. I thought I'd solved it because the sponges blocked up a lot more slowly. However, when I did get round to cleaning them I was back to square one with the gunk thing when I lifted the sponges out, gunk all over.
I had to have a rethink and make it better, more workable. The Eureka moment came when I realised it would be better if the sponges were inside something, something I could just lift out on maintanance day, no gunk or anything. And that is when my bucket idea came into play.
It is brilliant.
My point is. At this moment you are in the very early stages of where I was. Trying to work things out. I got there in the end through trial and error. So will you.....but i'm afraid you gotta go back to the drawing board regarding your current plan!