I have a 29 gallon BC with the middle chamber as a fuge and it works great.

I also keep the water level at the top of the little window on the side.

I did not take out any walls though. What i did was put rubble in the bottom of the middle chamber up to the bottom of the back window and put chaeto right in front of that little window in the back. Then i stuck an LED puck light in that window.

I took out the BC filter on the far right chamber and put a bag of carbon in it with a heater. The third chamber is normal with no mods. In the middle chamber there is that little grate on the top... I put some filter floss on top of that to get the particles before it does down to my fuge.
I hope this helps Good Luck!