You might be better off going to 160x160mm square section for the reservoir, if you're prepared to build it yourself. It will take up very little extra floor space, and let you use 3x80mm fans.
Using 6" pipe (150mm diameter) is going to really restrict your design options. Even a small size increase could make a big difference.
If you're determined to use 6" pipe this might be a better idea. 30 second drawing, but the red arrows show the air flow and the blue the water. The right hand box is the fan, and the left hand one the bong section.
If at all possible you want to minimise the constrictions/seperations etc. in the air flow. This would probably take up to about a 120mm fan without too many changes.
