You should be aiming for water to flow in a single direction through the system. Dirty water flows out of compartments, past the filter wool, through the filter, then back to the compartments. Where you put the HOB kinda depends on where the gaps are for water to flow which I can't make out in the pic. You have to force water to go where you want it to others it will just take the path of least resistance. If the uptake and outflow of the HOB at in the same section, water will circulate mostly in that area.