Hey Kevin,
Sorry , haven't looked at this thread for a while.
Tobes is quite correct : The flow should go over the DSB and not through it.
Don't know what I was thinking ...
Anyway, if you wanted to still go with a DSB in the back of the Boyu you should perhaps rather look at a partition which you can put into the chamber. The sides of the partition wall should go all the way from the bottom and come to within 2cm or so from the top.
I can't remember how the flow from the second compartment to the third compartment goes. If it comes in from the top, then it's fine. It will flow over the DSB and over the partition wall and then out to the fourth chamber. Hope this makes sense ?
Your only trouble will be in sticking a partition into the chamber while your tank is already filled with water. Only way I can see of doing this is to use either super glue ( the gel type) or Prattley putty. Line the sides of the partition wall and then stick this into the chamber.
All of this may be more trouble than what it is worth and you would be constricting the flow slightly.
Better idea may be to convert the third chamber into an algea scrubber.
Just a thought.