I tried cooling a 400W HPS 125mm cooltube with a 125mm 240VAC PC fan with no luck, tried two - back to back so both pushing air through tube mounted together, tried pulling air through with both mounted together, tried one either end of the tube push/pull which worked a little better but no combination could cool it properly. I got a 4" vortex fan with great results, it's quieter also.
3' x 3' x 8' veg cab temp went from 36 degrees down to 24 and the exhaust from the tube went from 48 degrees to 29.
Dont even bother if you have a carbon filter or ducting over a meter they just cant move the air.